"I would argue that marriage is a personal contract between two people, and the state has no business interfering with that personal contract."
Perfectly said. That's why I disagree with lots of people who oppose gay marriage because their beliefs are against it. Even if it is "wrong," it's not the government's job to decide what's right or wrong for individual people. The thought of putting ketchup on cake is abhorrent to me, but I wouldn't want it outlawed, and if it was, I'd be first in line to vote to legalize it.
I think that it's clear to most of us that gay people should marry if they want, and I would hope it's clear to all of us that women should vote, racial minorities should vote, no one should be slaves, et cetera.
But they have these votes because people see it differently than me and perhaps you do. They think that the government should be a moral guardian of sorts, and that their interpretations of morality say that this minority might not actually have the right to do certain things. To expound into more of a devil's advocate argument, children under a certain age shouldn't vote, nor should they. Why? Because they are of lesser intelligence. At different times in history, and even now, people think that minorities, the other gender, or different sexualities are less intelligent, or in this case, simply lesser than their own, and thus shouldn't be given the same rights and opportunities.
I could explain why, but I'm not a psychologist, and even though I'm kind of a writer, doubt I could make it coherent. I think it has to do with self-righteousness, or maybe a resistance of change in favor of the stability of tradition, but hey, damned if I know.
The point is that, even for things that seem obvious to some, even unalienable rights, to ensure these things you need to work to get them. Maybe that's wrong, but it's how it is. Liberty often needs to be obtained through campaigning and legislation, if not all-out revolution.