DOMA was meant to do that and it got struck down. I’m not sure what else you’d want.
He would do far better to advocate that the government should not be social engineering, which “gay marriage” is in spades.
Paul opposed DOMA.
Paul is running for federal office, he needs to want to make it against the law and policy at the federal level, goals that you oppose, as does he, as do libertarians.
Paul on same sex marriage
Gay marriage, for instance, is one issue on which Paul would like to shake up the Republican position. “I’m an old-fashioned traditionalist. I believe in the historic and religious definition of marriage,” he says. “That being said, I’m not for eliminating contracts between adults. I think there are ways to make the tax code more neutral, so it doesn’t mention marriage. Then we don’t have to redefine what marriage is; we just don’t have marriage in the tax code.”
http://www.nationalreview.com/articles/342813/rand-paul-s-big-fight-robert-costa?pg=1