Don Herzog over at Left2Right has a nice post on the supposed reasons to oppose gay marriage and their mutual incompatibility. He says there are three basic arguments:
1. Family law belongs to state governments. But it’s outrageous for state courts to rule that gay marriage is constitutionally required. That decision belongs to the people or legislatures of each state.
2. “Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State.” That’s Art. 4, sec. 1 of the Constitution. If one state marries gays, it looks like other states would have to recognize those marriages.
3. Same-sex marriage is wrong, period. Marriage must be between one man and one woman.
Now if you are a strong proponent of federalism, then (1) makes sense. However, many people hold (3) but then try to argue (1) while at the same time saying there should be a constitutional amendment to prevent the situations arising from (2).
In fact, I was at a party last year where I was arguing this case with someone and it was clear that they thought (3) but instead were making some specious claims about the state having a vested interest in making babies. When pressed further on that they retreated to (1) with a sprinkling of (2) for justification. I viewed it as a minor success that I could talk them out of the constitutional amendment.
Of course, an arsenal of counter-arguments is only half the battle. But arguing the benefits is much easier, I think. Unless you’re CNN of course. Sometimes the Daily Show makes me hate the world.
An Ergodic Walk 
It would be fascinating to examine the rhetoric that surrounded the Loving v. Virginia decision that struck down anti-miscegenation statutes. I bet there were calls for a constitutional amendment then. I know that a strong majority disfavored interracial marriage. It’s an interesting and pertinent parallel, no matter how much people try to deny it.
As a poster on that blog noted, the best news coverage on the air right now comes from a comedy show. And that’s scary.
If you are a proponent of federalism, then you think each state should make its own rule, and (1) does NOT follow. Nothing about federalism should prevent a state court from ruling that a state constitution prohibits or mandates a certain kind of law. Indeed, federalism means RESPECT for state court judgments, not overturning them.
Number (2) does not apply here; the public policy of states is a good enough reason not to grant faith and credit, and Congress can (and has) said that no state needs to recognize out-of-state gay marriages. This is settled case law, because there are already differences in which breeder marriages are permitted state to state. A state does not have to recognize an out-of-state marriage which is contrary to its own public policy.
Number (3) isn’t an argument at all, but I would point out that marriage existed before the state did; states may choose to recognize a given marriage, but it isn’t the government that makes a marriage, it’s love.
I probably should have said something like “federalism and a desire to reflect the local will of the people.”
Not that I think any of these reasons are good reasons to oppose gay marriage, mind you. Or that there are good reasons.