Article I Section 10 Clause 1 of the Constitution holds that “No State shall…pass any…law impairing the obligation of contracts”
I’m not sure how this has been traditionally interpreted (but I gather that it has traditionally been read to apply to existing contracts and not to blocking the formation of new contracts), but it seems to me that a reasonable reading could get you the result that states can’t forbid gay marriage contracts (not, of course, that the federal government couldn’t).
I gather the reason the gay marriage cases have been argued they way they have (under the 14th amendment’s cue process clause) is because the Supreme Court has fond that marriage is a fundamental right protected by that part of the Constitution and any restriction of this right will have to sever a clear state interest and the law must be narrowly tailored to achieve that end and only that end. This is Loving v. Virginia if you want to read the decision. This forces opponents of gay marriage to argue that it would harm the state in some way (which is what they normally do) or that the right to marriage is really just a right to heterosexual marriage (which seems to be a weak argument, but they make that too).
Just my two cents (Obligatory: I am not a lawyer and this should not be taken as legal advice)