My wife and I were married in Pennsylvania. If we moved to New York, I am confident that New York State would recognize our marriage and allow us all the rights of married people, such as joint tax returns, inheritance, etc. The reason I am confident is that the U.S. Constitution, from the very beginning, required states to accord "full faith and credit" to the acts of other states. Since Pennsylvania married us, New York cannot refuse to recognize our marriage.
Even before the Constitution was signed, states (or colonies) had their own criteria for who could get married, and still do. In most states, for example, the minimum age to marry is 18, but it is 19 in Nebraska and 21 in Mississippi. If a heterosexual couple marry at 18 in Alabama, and then move to Mississippi, the Full Faith and Credit Clause requires Mississippi to recognize their marriage.
The same, of course, applies to a couple married under the laws of New York who move to Pennsylvania. And since New York permits gay marriage, the couple might be two men or two women. If this argument is correct, Pennsylvania, and every other state with a state law against gay marriage, can decide who can get married in the state, but not who is married in the state.
In the recent case of Windsor v. United States, the Supreme Court ruled that Section 3 of the Defense of Marriage Act (DOMA), was unconstitutional. Section 2 of the same act, however, says that "No state... shall be required to give effect to... any record of any other state... respecting a relationship between persons of the same sex that is treated as a marriage under the laws of such other state..." If the Full Faith and Credit Clause of the Constitution applies, it will require Section 2 of DOMA to fall as well.
The practical consequences of allowing states to refuse to honor marriages performed according to the laws of other states would be severe. Would this mean that divorce could be achieved simply by moving across a state line? If one spouse lived in a state permitting gay marriage and the other did not, would the former be married and the latter unmarried? This way lies legal chaos, and does not comport with the dignity the Supreme Court requires for gay marriage.
It should be noted that legalization of gay marriage does not require anyone to participate in one. There are conscientious people who do not favor the idea. No religion can be required to marry homosexual couples. All that is at stake is the liberty of homosexual couples to marry if they wish.
Once people get used to the idea that there are gay married people in their state, the terms of the argument shift to an economic base. Why should the resorts in the Pennsylvania Pocono Mountains lose gay marriage business to similar resorts in New York? Why should Pennsylvania lose sale tax to New York because of this law?
Just as casino gambling spread from Las Vegas and Atlantic City, New Jersey, to most of the country, gay marriage will spread for the same reasons. Gay marriage is coming to a state near you, even without a continuation of the shift in sentiment seen so strongly and so recently.