Anyone who knows me knows I oppose homosexual marriage, and this probably has to be handled at the federal level because even one state allowing homosexual marriage can probably force every other state to accept it, under longstanding precedents.
The contracts clause is in Article I, Section 10, which reads as follows: “No State shall ... pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts...”
The alternative to considering marriage to be a contractual obligation covered by this clause would be chaos with people not knowing if their marriage performed in one state would be valid in another state to which they might move. Less than half a century ago, this became an issue when some more conservative states didn't want to recognize the “no fault divorce” granted by states with looser laws. The stricter states were forced to recognize divorces granted in more permissive states. Also, states which prohibited interracial marriages were first forced to recognize interracial marriages performed in other states and then forced to perform interracial marriages themselves.
This is not a new issue — there are multiple reasons why Utah could not be allowed to enter the Union before it outlawed polygamy, one of them being that every other state probably would have been forced to recognize polygamous marriages in Utah. Even apart from that extreme case, this came up in the early 1800s with questions about the legitimacy of marriages to first cousins or states having to decide whether to allow remarriage of people who had been divorced under the laws of a different jurisdiction where divorces were granted on grounds that other states refused to accept.
Personally I can see some wisdom in letting this get handled as a states’ rights issue, but I see no way to do so because of the contracts clause and court precedents relating to marriage that go back long before modern judicial activism. Much will depend on what the Supreme Court does, but I'm afraid this is probably going to end up being an “all or nothing” issue with homosexual marriage allowed in all states or no states.
I'm not advocating this, but I've read about some Conservatives and Libertarians pushing for a third way which goes down the privacy path. Basically, it would forbid the government from asking for or tracking anyone's sex or sexual orientation. It basically tells the government it is none of your business. Thus, gay marriage or whatever wouldn't be expressly forbidden because the government could not ask either party what their sex is, it would be a blind contract between two individuals.
It also sounds like it would be a complete cluster the day after it is implemented when you then put into the mix religious ceremonies and people trying to carry forward that 'blind' rule to religious institutions as well.
It seems, the definition that has crossed almost all cultural, religious, and national lines for thousands upon thousands of years is the easiest to manage and maintain. You can't just go say a foot is 13 inches any more than one can change the definition of fundamental legal terms without a butt load (no pun intended) of unintended consequences.