I don't believe in gay marriage.
I don't believe in traditional marriage, either.
Government sponsorship of particular chosen relationships through award of special rights and monetary rewards is pernicious. Like most government action, it perverts people's choices. I am only married today because I had a reasonable fear that if I moved my boyfriend in, some judge might decide my kids would be better off with their father. The only reason a judge could do that is because government has designated marriage as an acceptable relationship between men and women, one that does not call into question a mother's or father's fitness to have custody of their children.
Because I am heterosexual, I had the privilege of getting married to avoid that possibility--government gave my relationship its imprimatur and made my ability to continue parenting my children less assailable on moral grounds. That's wrong: I don't deserve special rights because I'm heterosexual, and my ability to parent does not depend on whether I'm married or just living with my chosen partner. It's just an easy shortcut to judge me by whether I'm married; it doesn't tell you much about me, certainly not enough to judge whether I'm a fit parent.
That's why I want government out of the marriage business all together. Maybe a simple registry whereby people could record their commitments and the ending of them would be okay, so long as there were neither rewards for registering nor punishments for failing to, and religious marriage is a matter for the particular religions (as it always has been anyway, each religion setting the rules for what marriage requires of its members), but in general it's rarely good when government interferes in family relationships, so let's keep it out of this one too.