As a Constitutionalist, Dr. Paul's solution to issues like gay marriage is simple and principled: it's no business of the Federal government. As a libertarian, his broader answer is that these issues are no business of government at any level. But since Ron's running for President and not for the statehouse or governor's mansion, it's ultimately not for him to say what states should or should not do.
There are, of course, a few inherently federal issues on which a President and a Presidential candidate must take a stand--domestic partner benefits for Federal employees, for example. But only one of these issues has yet attracted significant public attention: what do do about gays in the military, the infamous "Don't Ask, Don't Tell Policy."
Here is Ron Paul's most concise statement on the issue, from the Republican Presidential Debate, June 5th, 2007: "We don’t get our rights because we’re gays or women or minorities. We get our rights from our creator as individuals. So every individual should be treated the same way. If there is homosexual behavior in the military that is disruptive, it should be dealt with. But if there’s heterosexual sexual behavior that is disruptive, it should be dealt with. So it isn’t the issue of homosexuality. It’s the concept and the understanding of individual rights. If we understood that, we would not be dealing with this very important problem."