As a starting proposition, I feel strongly that gay couples should have the same rights to marry as any other couple.
That being said, I think that other things being equal, the combination of a man and a woman as parents probably has the greatest potential for raising happy and healthy children, because I believe the gender roles we as a species have evolved over the course of millenia serve a legitimate purpose (just as they do in other species). That is not to say that a gay couple can't also be great parents, and better than many heterosexual couples, but I do think that other things being equal, a heterosexual couple is better suited to raising kids than a gay couple. Given that one of the principal reasons for marriage is creating a structure for parenting, I can certainly understand the views of someone who believes that only heterosexual couples should be "married" in a legal sense. I don't happen to agree with that view but I don't see it as necessarily homophobic.