Nobody can tell, and this one really is a crap-shoot. This answer is a bit long and, therefore, is written with a mind to:
- Explain American law and jurisprudence to our non-American friends.
- Explain some of the finer points to Americans who either haven't cared until now or who generally don't pay much interest to legal politics.
- The usually interested who might want to see what the sundry issues will be that this court must deal with.
Key Words & Phrases:
- SCOTUS (Supreme Court of the United States)
- Bill of Rights (colloquially, the first ten amendments proposed and adopted directly after the ratification of the Constitution in order to more clearly define civil rights within the USA)
- DOMA (Defense of Marriage Act, passed in 1996, denying gay marriages on a federal level or recognition and permitting the states to decide for themselves and NOT have to recognize those of other states/nations).
What conflicting court issues are being combined into one "hearing" for the Supreme Court:
- From California: Lawsuits that have worked their way up from the State Court around Proposition 8 in that it is (a) against the US constitution to deny people their rights on the XIV Amendment and (b) it is highly unethical to grant civil rights and take them away.
- From New York: The federal government is denying benefits to survivors of spouses who earned those benefits. There are a few of these, but the essence is that the Federal Government SHOULD recognize Canadian marriage licenses and the marriage licenses of the states who issue them.
What we have are:
- Constitutional stricture, ambiguities, and conflicts.
- Evolving contemporary ideals.
- US Supreme Court ideologies regarding proper government and human behavior.
Key Justice to watch:
- Anthony Kennedy (Associate Justice) - The court is pretty fractured. Without a doubt, Kennedy is the most important human being when it comes to gay rights LIKELY IN THE HISTORY OF THE USA (exponentially so if he comes down in favor of gay marriage).
- John Roberts (Chief Justice) - I will repeat this several times, he may want a longer legacy, and it appears he's willing to see this from the long view.
-- Constitutional Stricture, Ambiguities, and Conflicts --
- The Full Faith & Credit Clause - ("FF&C") demands that states respect the laws and institutions of other states; it does NOT permit the federal government to limit this clause or define it. It says what it means.
- IX Amendment - which states that people have more rights than those simply spelled out in the Constitution. There are others that have not been defined or imagined. It doesn't say who's to extend or define them, so it's left to the individual states, Congress, and the SCOTUS to define who those people are, what those rights are, and how they are applied within their jurisdiction.
- XIV Amendment - demands that all citizens be treated as equals before the law and not be denied the rights and immunities enjoyed by the citizens of the United States.
- The Necessary & Proper Clause - ("N&P") which grants the US Congress the power to enact laws that are "prudent and necessary" to exercising the powers defined in the Constitution and those that the SCOTUS interprets are within its prevue based on the ambiguities of the Constitution.
- Does the Congress have the right under the Necessary & Proper Clause to limit the scope of the Full Faith & Credit Clause?
- Do gays apply under the XIV Amendment's wording, despite the fact that this is CLEARLY not what the framers of that Amendment wanted?
- SHOULD gays be treated as a suspect class?
- SHOULD the Supreme Court get ahead of this issue and on the "right" side of history before history outruns the Court?
- Do we need to concern ourselves with original intent, since clearly, we have evolved ethically beyond what they wanted in the first place?
- Is this right for the country?
- Will a Constructionist vote in favor of Full Faith & Credit and strike down DOMA or see this as a larger "interstate" issue and uphold Congressional powers?
- OUTCOME ONE: Side with the Congress, hold that gays have no specific human rights pertaining to their sexuality, uphold DOMA (under the N&P Clause), restrict FF&C Clause, and leave current law in place. States WILL still keep their gay rights laws, but the Federal Government and the other states will not have to recognize. This will be the law of the land, then, until Congress "evolves" and passes a gay civil rights act AND/OR the states individually pass these acts AND/OR the SCOTUS is altered by retirements (and likely will be the time Obama leaves in 2016). Not as much of a disaster as some gays make it out to be: this is merely a continuation of the status quo and the tide is turning no matter what the Court decides. My best "Nate Silver" odds: 4.5/10
- OUTCOME TWO: Call DOMA a congressional over reach beyond the N&P Clause, enforce the FF&C clause nationally, and expand the IV Amendment of the Constitution to include gays and lesbians (and likely transgender) thus being the largest civil rights expansion in US history since the 1960s. States and the federal government will ALL be required to issue and recognize gay marriage (likely within a reasonable time-frame to work out the logistics). My best "Nate Silver" odds: 5.5/10.
- OUTCOME THREE: Call DOMA a congressional over reach beyond the N&P Clause in relevance to the states and enforce the FF&C act nationally BUT permit DOMA in the federal sphere because the federal government DOES have the right to govern its own actions. Highly unlikely. Courts don't like creating a gelatinous legal mess. My best "Nate Silver" odds: 1/10.
- OUTCOME FOUR: Individual decisions based on the lower courts rulings (i.e. "California's Prop 8 is thrown out, but the lower court's ruling in NY stands."), also unlikely. My best "Nate Silver" odds: 1/10.
-- Evolving contemporary ideals --
The people who sit on the bench aren't dumb. I respect ALL of them (well, except maybe for one). They are all profound thinkers, fair minded, and very intelligent. Each of them comes to his or her ideologies through very sound and rational means.
That said, ALL of them are completely aware that the tide has turned on this issue and only a moron would predict that twenty years from now, the USA won't have gay marriage -- barring an invasion from Mars, the sudden appearance of Christ descending from the clouds (above the USA, of course, we're Jesusland), or a violent revolution destroying the nation -- we all see this coming.
Many justices are concerned with evolving ideals concerning gays; some are staunch supporters of traditional ethics; others may believe that gays deserve to marry but cannot bring themselves to "reinterpret" the Constitution to make that happen.
But there is the fact, that the court will want to be on the right side of history and this will be a heated debate in their chambers. Nobody wants to be the "justice" who wrote another Dred Scott decision and ridiculed for eternity.
-- US Supreme Court ideologies regarding proper government and human behavior --
The court has to balance
- One justice will be strict interpretationists and not judge the morality of the behavior. (Scalia).
- Two are more or less guaranteed to see this through the lens of their social and religious values and judge it not worthy of protection. (Thomas & Alito).
- The Chief Justice will judge this through his views of the "nature" of the Constitution and may want HIS tenure as Chief Justice to be marked by being on the "right" side of history when he knows full well that times are changing (to avoid being a "Dred Scott" chief justice ... but may otherwise be hard-pressed to justify it by wording or his personal beliefs. (Roberts)
- One is very much in the middle and may side with the "left" because he -- like Roberts -- won't want to be on the wrong side of history. (Kennedy)
- Four others are staunch liberals and will want -- both based on their Constitutional interpretation and personal ideologies -- to extend rights to gays. (Ginsberg, Breyer, Sotomayor & Kagan)
- Antonin Scalia - I actually respect this man. He makes decisions based on a fundamental ideology that basically runs on strict interpretation ("constructionism"). I don't like his philosophies, but I respect the man behind the ideology.
LIKELY: On its own without the Congressional act, he'd strike down the state's refusing to recognize gay marriage licenses out of state -- he's that much a Constructionist, but he's also likely to see this as a N&P power and uphold DOMA under Congressional purview. No grounds for IX or XIV Amendment extension.
- Clearance Thomas - Again, very conservative constructionist. Not a fan of this guy, but that's all I'll say. He will not vote in favor of extending the XIV Amendment to gays.
LIKELY: Uphold N&P on its own, but likely to do so because of a distaste for the moral implications of gay marriage. No grounds for IX or XIV Amendment extension.
- Samuel Alito - Not a chance. Bush picked him for good reason: he's the most aggressively Conservative of the bunch. Where Scalia & Thomas are conservatives but will base their interpretations on fundamental Constructionism, Alito will allow his conservative stripes to influence him on what he sees as "moral" grounds.
LIKELY: Uphold N&P on its own and won't budge on the moral grounds and opposition to gay marriage. No grounds for IX or XIV Amendment extension.
- Ruth Bader Ginsberg - I could say basically the same thing about all three liberal justices: that is, they are ideological liberals in Constitutional interpretation and in social values. She wants this as the jewel in her crown before retiring. She wants to make history on this one and sees herself on the right side of history. PREDICTION: She'll retire after this decision.
LIKELY: Absolutely strike down DOMA as a Congressional over-reach; impose FF&C and extend the IX or XIV Amendment to gays.
- Stephen Breyer - Very progressive. He believes the job of the SCOTUS is to make the laws better for humans. He's willing to "reinterpret" to make a better nation.
LIKELY: Strike down DOMA; impose FF&C and extend IX or XIV Amendment to gays.
- Elena Kagan - No doubt, she's a liberal. She was put on the bench specifically because of her closeness to the Obama cabinet as an answer to Alito. She's a liberal activist judge who will likely see this as an IX and XIV Amendment issue.
LIKELY: Strike down DOMA; impose FF&C and extend IX or XIV Amendment to gays.
- Sonia Sotomayor - Yes. Sotomayor. We have no idea how she'll vote. She's a staunch liberal, but she's surprisingly willing to buck the trend. Since her ideology may permit expanded Congressional power to do such things, she may well vote to uphold DOMA despite personally disagreeing with it. Call her the Scalia of the Liberal side: she's got a formula and is likely to let that formula rule her decisions more than ideology.
LIKELY: Strike down DOMA; impose FF&C, extend either IX or XIV Amendments to gays.
- Anthony Kennedy - He's a centrist-liberal at heart, but he carries a lot of weight of the wording of the Constitution. He's actually very annoyed at the power-grabs by the Federal Government and this will weigh heavily on him. He's also been really pro-gay rights and crafted several legislative decisions in the favor of gays. Make no mistake: Kennedy will be the deciding vote in this case.
LIKELY: So hard to predict, but given his track-record of expanding and protecting gay rights, I think he'll strike down DOMA, impose FF&C, and extend either the IX or XIV Amendments to gays.
- John Roberts - I think he's becoming a social moderate at heart and even more importantly, he's feeling the weight of being the Chief Justice at a time when civil rights and our social mores are changing. He wants to be on the right side of history and will be fearful of being "that Justice" that history looks back upon and thinks of when prejudice decisions are mentioned. Everybody fears for his/her legacy, Roberts included.
LIKELY: Again, hard to predict. I think he'll possibly keep DOMA because he has no issue with growing federal power. I don't see him extending the IX or XIV Amendments to gays.
- I hate making predictions because I hate being wrong. But I'll take a risk. Let me say, I'm not a betting man on this. I'm like 55/45 and won't be surprised either way when it happens.
- I predict that DOMA will be struck down. I also predict this court will not want a divided decision and will be concerned for its prediction effecting the Federal Government. I hope it extends rights to gays, and I cannot help thinking that that "hope" is clouding my judgement, but in the end, I think it'll come down in favor of gay marriage and require both the states and federal government to recognize it.
- In a FIVE TO FOUR decision in favor of gay marriage:
Kennedy, Ginsberg, Breyer, Sotomayor, Kagan: I think Ginsberg will pen the decision as something she's wanted for several decades and retire a proud woman. All five speak, especially Kennedy.
Roberts, Scalia, Alito, Thomas: Scalia (maybe Alito) will pend the dissent. Scalia will likely rage and give one of his legendary rants. Thomas will not say a thing. Roberts will likely give a mixed dissent wanting to leave passion out.
- In a SIX TO THREE decision in favor of gay marriage:
Roberts, Kennedy, Ginsberg, Breyer, Sotomayor, Kagan: I salivate at this decision. I actually dreamed about it last night (no shit). I won't hold my breath, but such a decision carries a powerful weight behind it. Legally it's the same, but socially, it is a sledge-hammer to conservatives. Ginsberg writes the decision. Roberts delivers.
Scalia, Alito, Thoma: same as before, a very vitriolic dissent from Scalia and likely Alito.
-- Closing --
The really interesting thing is that -- while I'll be THRILLED with a decision in favor of gay marriage -- I won't be one of the people wailing in the streets calling this a "travesty" and running around simmering in a stew of piss and vinegar. I will be saddened a bit, but I really see this as a long fight and no matter what the court decides; we're at least two full decades away from the social change I want to see.
Even if the court decides against gay marriage and upholds DOMA, the fact still stands that on a local level, we saw the first cracks against homophobia on a political level. And if the proponents of gay marriage lose in 2013, then it just means we'll be fighting state by state in the next several years. California, Oregon, Rhode Island, New Jersey, Delaware, and Hawaii are next.
With California, Oregon, New Jersey, and Hawaii being the ones already significantly over the tipping point to amending their constitutions democratically. While I hate putting human rights up to a vote (we're a republic, not a direct democracy), there's a certain joy in seeing conservatives lose these votes.
Answer by William Petroff, Student of Political Science
Dan Holliday has a rather robust answer here, and I agree with a lot of it. I won't bother rehashing the legal stuff, because he does good job with it, so I'll simply leave what little I have to quip about it to footnotes and commence with the rundown:
Scalia: He'll almost certainly vote to uphold DOMA. I don't think he'll bother with the Necessary and Proper clause and instead simply use the Full Faith and Credit clause as the vehicle to impose the legality of Congress's power to legislate the issue of marriage. For the additional justification, in order to sidestep the 10th amendment and find solid ground for DOMA's Section 3, I think he'll try to put the screws to Roberts and the liberal wing by trying to claim that Congress can implement a federal definition of marriage because of the Spending power and the vast tax implications marriage has after the Affordable Care Act ruling. Unlike Dan, I'm fairly certain that, absent DOMA, Scalia will still vote to uphold Prop 8, despite how much of a strict-constructionist he is. He's never been a fan of extending Equal Protection rights based on gender or sexual orientation (because he's a strict-constructionist) and he's generally been amenable to allowing states to ban activities they deem to be immoral. 
Thomas: This is a justice who doesn't really care about anything outside of his judicial philosophy. He'll almost certainly vote to uphold DOMA, though there are two paths I think he could possibly take. One path is simply voting along the same lines as Scalia, allowing for Prop 8 and using something like Pacific Employers as his legal justification to buttress the argument if DOMA goes down. Another path he can take (and Thomas does like to go his own way) is to disavow the entire notion that marriage is a constitutionally protected right, which, coupled with the fact that he wouldn't apply strict scrutiny, probably gives him enough wiggle room around the 14th.
Alito: He's the conservative man's conservative on the court. Like Dan said, he's not shy about letting his conservative leanings come out on social and moral issues. He'll vote with Thomas to allow both.
Ginsburg: She's taken on John Paul Steven's mantle as the liberal rock of the Court. There's no way she let's either law stand, using the 14th amendment to strike down Prop 8 and the 5th amendment to strike down DOMA (after having established a protected class in order to be able to render some level of scrutiny above the rational basis test).
Breyer: He's (almost) every bit of the liberal that Ginsburg is. He'll strike down both and join Ginsburg in looking to establish a suspect class.
Kagan: She seems to be a little less liberal than the two justices above, though not by much. But on the important questions, there's no doubt that she's going to join them.
Sotomayor: I agree with Dan's comment that Sotomayor is a little bit more to the center than the other three members of the liberal wing (and certainly a bigger believer in the robustness of government power), but I don't for a second think she's going to do anything other than strike these two laws down. You can believe that Congress has the theoretical power to make DOMA a law and still believe that law to be unconstitutional.
Kennedy: I don't think that he's as much of a wildcard as people believe, but I'm still not confident enough to put him in with the liberal wing just yet. I disagree with Dan that Kennedy is a centrist-liberal, rather that it's a tale of two Kennedys: one that tends to align himself with the right when it comes to things like gun control or religion in the public square, but, at the same time, has liberal tendencies when it comes to issues like gay rights and abortion. This is the man who wrote Romer and Lawrence. I find it hard to believe that he'll do something other than strike down both DOMA and Prop 8 (though DOMA is certainly more likely than Prop 8), and I think he may even go ahead and invoke suspect class. But since it's Kennedy, I'm not going to believe it till I see it.
Roberts: A year ago, I would have said that he'd be with the conservative wing without flinching. Now, I'm not so sure. It may still be some level of bias after the Affordable Care Act ruling, but I'm viewing him more and more as aligning himself with the institution of the Court more than a particular philosophy these days. Don't get me wrong, I still think he leans more to the conservative side. But where I once thought that he was right-of-center, I now see him more as a center-right. Ideological philosophy aside for a second, there are a few things at play for him that I don't think are in play with the other justices:
- As the Chief Justice, he's prone to be more concerned about the power and perception of the Court itself, which has been waning over the past few years. He's clearly looked to depoliticize the Court (Citizens United not withstanding), and he's tried to form opinions around as large of a consensus as possible.
- To the end that he's tried to form as much unanimity as possible, he's preferred an incremental approach, looking to decide cases as narrowly as possible (again, Citizen's United not withstanding).
I'm optimistic that DOMA is going down, and am cautiously optimistic (65-75% range) that both laws will get struck down. I'm less confident (55-60%) that both will be struck down in a somewhat convincing manner (i.e. larger than a 5-4 decision).
 And this is one of the few legal area in which I'll disagree with Dan's answer; the Full Faith and Credit clause doesn't exactly mean what it says (despite the fact that we may want it to). It is established doctrine that there are limitations on the idea of "Full Faith and Credit," in much of the same ways that there are limitations on the freedom of speech. The long held standard is that a person can't argue that a state must substitute the statute of another state if it already has a conflicting statute on the books (this idea comes from a 1930s Supreme Court decision in Pacific Employers Insurance Company v. Industrial Accident Commission). States used this tactic in order to decline the acceptance of interracial marriages from other states before Loving and this limitation on "Full Faith and Credit" is still, as far as I can tell, the precedent. Of course that begs the question why was DOMA needed in the first place, but that's going off on a tangent.
 It gets a little trickier here because the Court has put a lot of restrictions on Congress' ability to use the Spending power as legal justification (South Dakota v Dole), and I'd be of the opinion that it doesn't pass the "germaneness" test, but I'm sure Scalia will find a way around that.
 Seeand .
 This is another area that I may or may not have to disagree with Dan on a legal issue. If Dan is talking about the justices using the 14th amendment to strike down DOMA, I'll disagree because the 14th amendment isn't a vehicle that can easily be used to strike that legislation down. It's an amendment that's geared around restrictions placed on the states, not the federal government. To get around that, you have to use a nice little legal maneuver the Court used in a 1954 case (Bolling v. Sharpe) about school segregation in the District of Columbia. The Supreme Court was in a bind since they couldn't use the 14th amendment to force desegregation, so they instead argued that it was a violation of the plaintiffs' 5th amendment rights and that (here's the really important part) "the concepts of equal protection and due process, both stemming from our American ideal of fairness, are not mutually exclusive." If, on the other hand, Dan is referring to Prop 8 when talking about the 14th amendment, then I clearly agree and apologize for making the reader read all of that. It's an equal protection issue, but it's just not an "Equal Protection" issue.
 Breyer, though, has had odd moments where he's joined the conservative wing (Scalia, Thomas, Alito, and either Roberts or Kennedy) on a handful of cases over the years. That won't happen here, however.
 Roberts also tends to be in the majority (only Kennedy was in the majority more than him last term:). Maybe he likes being a winner, maybe it's just that he's as much of a swing voter as Kennedy, maybe it's just that the conservative wing has won more than they've lost recently; make of that what you will.
 I think Dan's right, Roberts certainly isn't looking to become another Tawny or preside over another Plessy. He wants the Court's image to be seen as unbiased and apolitical, and that's certainly had an impact on the way he's looked to influence decisions.
More questions on Same Sex Marriage: