I don't understand how state legislation is any better than federal marriage legislation. If you're going to tell the government to butt out, just stick to it instead of tacking on an unrelated states rights issue.
Unrelated? It has everything
to do with state's rights!
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
Since we're talking about the Supreme Court, we're also talking about the Constitutionality of gay marriage (or, in my opinion, any marriage). The Constitution does not have any provisions or clauses to indicate what the definition of marriage is. Therefore, it should not be a Federal issue (and, in my opinion, any legislation on the federal level that uses that as a basis - for example, taxes - is in violation of this). Since it isn't a Federal issue, then it should be regulated by the individual states, if they even want to get involved at all (e.g. Oklahoma's current proposal to get the government out of marriage).
The rational behind this? We go back to the free market. Let's say gay marriage is approved in your state and you are so adamantly, rabidly opposed to it that you just can't stand the state and the people in it any more. What can you do? You can move to another state where the laws are different. It doesn't have to be just about gay marriage - perhaps it's the legalization of gambling, or gun control, or a mandate that everyone needs to have a rabid badger in their living room. And if a state finds that it's losing large portions of its population because of its nutty laws - and, likewise, losing the tax dollars to fund its nutty laws - they may work on changing it to attract people to the state again. In essence, it makes the states beholden to its citizenry to pass and enforce laws that the people of those states like.
If the laws were passed on a federal level, though, what are your options? The only ones available would be to suffer under the laws or move to another country. And from what I've heard, it is a lot more difficult to become a citizen of, say, Mexico, than it is to move from Washington to Oregon.
Back to gay marriage, the big question is whether or not the states have the right to legislate it, or if it's a federal issue as outlined by the Constitution. The Supreme Court may have a different opinion than I do (no, I'm not a Supreme Court justice...), but if the Supremes determine that the Feds do not
have the power to regulate marriage, then it very much becomes a states-right issue, thanks to the Tenth Amendment.