If, instead, you argue that the state has a compelling interest in homosexual marriages, then you have an argument. Scot consistently makes that argument, and it's a compelling one. In our society, there are plenty of people raising children as if they were both parents, even though biologically, they can't be. Assuming marriage affords protection to children and those who raise them, and assumingthat's in the interest of the state, then those people need the same protection that children raised by natural parents and other heterosexual couples receive.
Then, the anti gay marriage side would have to revert to an argument that same sex couples ought not to raise children. Some would use that argument, but it's losing ground quickly, as more kids are raised htat way, and no harm seems to come from it.
That's why I gave up opposition to gay marriage. I'm not prepared, in the absence of evidence, to tell Scot he can't raise children, and if marriage is important for children, then it's important for his children, too.