The argument would be that what you DO is inseperable from what you ARE, that the latter dictates the former. A diabetic doesn’t CHOOSE to go into a coma; he does so because he is diabetic. He may choose to inject insulin, which may stave off the coma, but even in that case, his behavior (the injection) is a function of his nature (being diabetic). Don’t shoot me; I don’t believe any of that claptrap. I’m just playing the Devil’s advocate.
The real answer to this specious defense is that we are not opposing homosexuality or even homosexual behavior. That will be judged by the Ultimate Authority. What we oppose is the false normalization of deviancy and the insistence that society grant it its imprimatur.
But two men choosing to marry is clearly different from a diabetic falling into a diabetic coma. One is a positive act of the will. The other is what happens when you do nothing.
And beside, the entire premise of civil rights is that people should be judged by what they do, not by what they are. That's why we don't look at race when we define marriage.
If they want to say that doing and being can't be separated, then they are making our argument for us. They are saying that homosexual rights follow from a completely different logic than civil rights. If they want to hang themselves with that argument, I say let them.