With regard to sex, Thomism is a question-begging exercise that puts a veneer of essentialist metaphysics over the question, “Why is God upset if humans don’t make use of their sex organs in accord with their primary and essential function?”
The metaphysics on offer to this question is the magician’s left hand distracting you from what is going on in the magician’s right hand.
What’s going on in the magician’s right hand is demographic maintenance. A religion like Catholicism survives by goosing the ideology of big families, and promoting it as dogma. To maintain 15-20% of the global population as Catholic, you’ve got to try to match the fertility rates of competing ideologies, such as Islam, which has a very high birth rate.
So the reason Thomists can’t reason their way out of the pretzel of their metaphysics surrounding sex is because it would cause the demographic decline of Catholicism globally. Thus the whole Thomistic scene is constricted, not by truth (real essentialism), but by demography (and the requisite fake essentialism necessary to support high birth rates among Catholics).
This translates into gay marriage as a threat, masturbation as a threat, gender equality as a threat, working women as a threat, divorce and pornography as a threat, etc.
The very same demographic concerns that got Thomistic sexual philosophizing going in the 14th century (the struggle to maintain population) are playing themselves out as a subtext to the contemporary freak-out over gay marriage and the revisiting of Catholic sexual practices.
Unlike the 14th century, it’s not global population experiencing downward pressure, but Catholic population. Thomists imagine that they’re doing metaphysics in a demographic vacuum divorced from history. They think they’re reasoning their way objectively to the truth, discovering what God wants done surrounding sex (for example). The truth is that their intellect project is driven by the will to replicate an institutional system. Thus in the arguments for keeping sex procreative, the individual will is derogated to the intellect, while the intellect is subtly directed to justifying the dogma of the institution, with the institutional will cunningly concealed.
That will is reproduction.
It’s why Thomism always manages to find exactly what it’s searching for. As if by magic, the conclusions somehow manage to track seamlessly with the dogmas of the Church. If they didn’t, this would happen globally:
Prometheus Unbound 
http://mixlr.com/marlen-vargas-del-razo/showreel/is-porn-freedom-of-choice/