The premises underlying Thomistic arguments against gay marriage are four:
- sex’s goal should be directed ultimately to reproduction;
- marriage is for the rearing of children and the union of man and woman;
- marriage is naturally a social institution for promoting the growth of families. It is not not naturally a private arrangement between two persons and God; and
- masturbation, anal and oral sex, contraception, abortion, and upending traditional gender roles cannot further marriage’s ends, which are reproduction, rearing children, bonding man to woman, and promoting the growth of families.
These are the premises from which Thomists shut down gay marriage. They believe that the above generalizations should never give ground to particular desires or circumstances. To do so is immoral.
But from my point of view, the big brain that God (if God exists) gave humans calls all bets off in terms of the sorts of generalizations one can make about how human beings ought to arrange themselves into a greater circle of love and orientation toward God.
It’s not gay orientation that’s the problem, but Thomistic orientation (which is focused on reproduction instead of love).
And Thomism doesn’t take into proper account evolution. Evolution is about what Walt Whitman called “freedom and variety.” It’s experimental; it’s about making it new. Experimenting with new arrangements is how evolution proceeds. The Thomistic definition of marriage is therefore just one more culturally conditioned way of being among many, not essential to the nature of human beings qua human beings. Thomistic essentialism could (if it wanted to) shift the definition of marriage to incorporate a broader range of possibilities for the institution, placing the focus on love, not reproduction.
But anywhere one starts reasoning on such matters–love vs. reproduction–entails question begging. It can’t be avoided. Arguing about metaphysics, essentialism, sex, and gender is not akin to doing math, and the axioms we start our reasoning with are not written anywhere in stone.
From my historicist perspective, Thomists have always made up their positions on sex and gender as they’ve gone along, favoring conservative and authoritarian values temperamentally, rationalizing them, and not acknowledging this fact. Aquinas did it, and contemporary Thomists do it. And when I’m countering the arguments, I’m making it up as I go along as well, for I have no ultimate ground (traditional or otherwise) for my reasoning to proceed either.
So it’s a competition of languages: the language of love and equality vs. the language of reproduction and religious authority. In a secular democracy with secular judges, it’s pretty obvious why arguments for gay marriage are working in the courts, and arguments based in Thomistic notions of “natural law” and tradition are falling flat.
Prometheus Unbound 
It is true that adoption makes mute the above arguments. However, this does not mean the marriage is about love. Marriage is a contractual arrangement that has its roots in treating women and children as property. Also the entire notion of state sponsored lifestyles or mandates dictating love should be abhorrent to modern liberal philosophies and thinkers.
The acceptance of gay marriage is a stop gap. The end result is the end of marriage as a state institution entirely. That said, we could not get the support of enough people to destroy the notion of “traditional” marriage if they were aware of where it would ultimately lead. The progressive Christians are (as they always are) patsies in the battle between theocratic state love licenses and chaotic individualist determinations of love. I thank them and their ideologies for their help in getting this far, and will continue to do so for another ten years or so, when they will no longer be needed and will be discarded as so much other obsolete rhetoric.
Ahh, moot not mute and one of those ‘the”s should be a “that.” No edit button for us grammar nazis to fix our typos?
No worries about the moot vs. mute error. As for your libertarian point, I think the distinction between a civil marriage license and a religious marriage is a meaningful distinction that needn’t be done away with. The state needn’t just absent itself from marriage arrangements altogether. It helps give standing (for example) to people who want to include their spouses on a health insurance plan. Absent some sort of official recognition, you could just put your friends on your health plan and call yourself married to them. It could get a little chaotic legally if there wasn’t an institution that recognized civil marriage arrangements.