The First Amendment to our Constitution immediately proscribes making laws "respecting an establishment of religion." No Constitutional scholar argues about the intent of the founders to allow people to practice any religion, or no religion.
A law doesn't have to name a religion to be about establishing religion. If a state passed a law mandating that everyone go to church every Sunday or face Mecca and pray five times a day, people would get the message. Likewise, laws prohibiting people from doing things like eating pork or burning a bible would say the same thing.
Since there is no scientific reason to declare a fertilized egg a person, or to declare for no reason at all the impropriety of people of the same sex getting married, one is left with the explanation everyone is reluctant to evoke, that the reason is purely religious.
Numerous states are trying to crush the ability to obtain a legal abortion, the most prominent currently being Texas. There is a certain thread of sameness in all the efforts. They are driven by religious politicians and organizations that have no problem wearing their godly hearts on their sleeves. They call abortion all sorts of things, mainly murder, but never come straight out and say what's really on their minds, that whoever they worship says it's simply wrong to have an abortion. Interestingly, the same deities also find sodomy abhorrent, and marriage only proper between a man and a woman.
Of course, a statement invoking religious necessity would be problematic. If religion were the driving force, then there would be no reason that atheists would have to give up either abortion or gay marriage. That would provide additional impetus for a phenomenon already occurring in the United States, a steady increase in atheism. There would have to be two sets of laws, one for believers, one for non-believers. Or would there have to be separate additional sets for religions that weren't Christian-based?
Isn't this conundrum the whole point of the First Amendment? The founders, essentially all believers in Christ, knew that there was little in Christ in most men, and so, ordinary men could not be trusted to treat their fellow men in a Christ-like fashion. In fact, my black Labrador is more Christ-like than any person I know. All she wants is to love and be loved, and I've never seen her do an unkind thing. Even the concept of the "Christmas spirit" shows how little of Christ lives in us when we have to invoke the holiday of His birth in order to get people to show a little kindness to each other.
I'm no theologian, but I'm not aware that Christ said anything about being gay or having an abortion. He was a guide and teacher, and would have tried to get people to do what was best, but it is not clear to me that He passed along the right to other people to judge and incriminate. I would never suggest that Christ would have favored abortion, but nor would he have favored the existence of unwanted children, probably the greatest tragedy in our society. And would someone of such empathy have stood between couples of like sex who were devoted to each other? I suspect that Christ would have equally loved people who didn't believe in him at all, as long as there was any chance of their redemption, and even then, I suspect that he would have welcomed them to heaven if they acted in a Christ-like fashion without ever having believed in him at all.
The First Amendment is not the only issue. What about "Full faith and credit," discussed in the first section of Article IV? States are supposed to recognize each other's laws. You aren't going to get pulled over crossing state lines for driving without a license if you have a legal license in your home state. A license is a license. How is a marriage license any different? There are qualifications for a driver's license just as there are for a marriage license. Why would states uniformly accept one type of license and not the other? Why, in fact, has this not been more rigorously tested in court? Not being a legal scholar, I'm not aware of the history of such challenges, but regardless, it's time for another.
One might delude oneself about the reasons for why laws that try to limit abortion or gay marriage are so popular in some places. All one needs to do to find out the real reason is to watch CSPAN and observe the antics of religious extremists like Congressman Louie Gohmert or Governor Rick Perry, both of Texas. These men are not merely religious; they are proselytizers. They will not be happy until everyone is as religiously driven as they are; ergo, the First Amendment. It is the reason we don't have prayer in public schools -- not because prayer is inherently bad, but because of the un-Christ-like ridicule to which the non-participants would be subjected. As long as people are just people, we will need the First Amendment to protect us not just from their natures, but from laws that have nothing to do with anything but their religious beliefs.