Based on what little I understand about the US Constitution, there's not a whole lot in there about morality. There's a lot of suggestions for how government is supposed to work. And a few bits about what rights people are expected to enjoy without fear of persecution. Or prosecution. But there's not much in there about how right and wrong are defined.
That's the kind of thing was left to the Bible writers.
If you're the type to thump your Bible (or Torah or Qur'an or whatever), then you may be able to make a moral argument against homosexuality. (You'd probably be kinda wrong, but you'd be welcome to try it.) What you can't really do is skim through the US Constitution and find an article on which to build a case that gay marriage is against the law.
On the contrary, you can pick any number of rights outlined in the Constitution and argue that they--as written--account for the right of people who share a gender to share some wedding vows.
That's more or less what I took from this segment Fresh Air ran yesterday.
I also took away that Prop 8 may ultimately be a good thing for gay marriage. Simply put, if you want to get a case to the Supreme Court to establish a principle as immutable law, you need a catalyst. Prop 8--while it was something of a bummer in the fall of '08--looks to be that catalyst.
While the Sups have been acting kinda strange this week, they'd be hard pressed to wholesale discard what appears to be a pretty obvious matter of constitutionality.