If he argued for it on grounds of being economically or socially discriminatory, then of course not. Marriage is only "sacred" in a religious context. Breaking marital vows through adultery is morally repugnant but has no legal implications in and of itself. As long as the argument for gay marriage is legal and not religious, there's no hypocrisy.
Are you sure you want to open up that argument because remember that legal arguments go both ways?
Anyway, this is merely a theoretical question I'm posing because I'm quite sure that you'll close the door or tell me that it doesn't exist when I try to enter it.
