It was a HUGE mistake for defenders of the traditional definition of marriage to base our defense upon a moral disapproval of homosexuality or whatever gays and lesbians do to express corporal intimacy. HUGE mistake. Colossal and Catastrophic. For it allowed and still allows our opponents to say ad nauseam that the only reason we oppose the legal recognition of ss'm' is that we are mindlessly and viscerally trapped in a homophobic animus, that we are, in other words, hateful, spiteful bigots.
Not only was it a gigantic mistake, it was and is and will always be a gigantic non sequitur as well. The question is not whether or no same-sex relations are immoral but, rather--as the marriage movement has realized much too late--, whether or no they can form marriages at all. The question is not, in other words, a moral but an ontological one. To see why this is so one need only look at the laws that criminalize incestuous marriages. Incestuous marriages are bad marriages, immoral marriages, probably even dangerously unhealthy marriages, and the law agrees that they deserve societal disapproval, but this disapproval is expressed not by non-recognition but by criminalization.
Whether same-sex "marriage" is immoral or no is utterly irrelevant to this entire debate. The relevant question is whether same-sex "marriage" is a possibility and as such deserves recognition. The argument that same-sex "marriage" should not be given legal recognition because same-sex relations are immoral is UNBELIEVABLY STUPID because it implies that same-sex relations can indeed (like incestuous relations) form valid marriages, and that is the very thing we are supposed to deny! You have to acknowledge the existence of something BEFORE you can make any moral pronouncements upon it. It would be like saying that the law should not give unicorns any legal recognition because unicorns are immoral creatures. Unicorns don't even exist. Ontology ALWAYS precedes questions of morality for it is the OBVIOUS necessary condition for such questions. The marriage movement has foundered on a philosophical fallacy so elemental that anyone who can't recognize it as such should not even bother taking Philosophy 101. They should instead try to make a living as pet rocks.