Except, for years now, that’s left us with the question about what happens when a gay couple marries in one state and travels/moves to another.
The Constitution has already solved this problem
Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State. And the Congress may by general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof.
States have to recognize what happens in other states. It is true for births, weddings, deaths and most any legal circumstance. It is becuase of this, the Full Faith and Credit Clause, that you dont need a new birth certificate each time you move to another state
You might say varying laws would cuase havoc, but look at prohibition. Many counties are still dry counties(for instance, Moore County, TN, home of Jack Daniels Distillery). But most of the country is not, and that works out fine. As far as I know, in most of them, you can leave the county, get drunk, come back, and not be arrested for acohol consumption. In some you can even bring the alcohol back.
For gay marriage, you could go to another state, get married, come back, and couldnt be arrested for being married. Getting married could still be illegal, but
being could not. That would essentially be a ex post facto law, which is also forbidden by the Constitution.
Simillarly, Some states have the legal driving age at 18, some at 16. As far as i am aware, a 16 yr old with a valid lisence cannot be arrested for underage driving in a state where the age is 18
to another point, why should it even matter to our government who marries who?