There is no secular argument of substance against legalizing gay marriage. Even if I didn't have my own personal reasons for supporting it, there is no way I could be opposed. Of course I am in favor.
Actually, I have a secular argument against gay marriage. I don't think that we should deny homosexuals equal benefits, which is why I support civil unions, but I believe you can give them equal rights without redefining an institution that for thousands of years and in just about every culture has been defined as between a man and a woman. For me, it's not a civil rights issue, it's a language issue. Give them the rights, but don't call it "marriage."
Then don't allow civil marriage in general and make marriage solely religious. Either everyone is allowed to get a civil marriage or no one is.