We know that evolution high-jacks organs all the time to different purposes depending on context. The first tongue may have had the singular purpose of tasting, then it got used by the cat for cleaning the pelt, then by humans for speech and sex, etc.
So nothing evolution evolves is “settled” in the sense that the way one generation uses an organ is the way the next generation ought to use an organ (think of the penguin’s wing, which started as a tool for flying, then got hijacked by evolutionary imperatives as a tool for swimming).
And now evolution has evolved the human big brain. The human big brain accelerates old-school biological evolution by creatively hi-jacking everything around it. That seems to be what a human brain is: a clever over-comer of natural states and functions by putting them to ends other than those they came into existence for.
So if God says, “I made pain in child-bearing a curse for original sin,” God also seems to say, “I made humans with big brains for figuring out how to get around this issue by injecting epidurals into the spine.”
And if God says, “I made the penis for ejaculating semen into the vagina,” God also seems to be saying, “And I made big brains for figuring out how to bypass traditional reproduction completely, and make use of sex primarily for pleasure, joy, love, and bonding.”
Thomistic ethical premises are grounded in a too-simplified notion of what can be read off of a fact of nature. Evolution doesn’t limit its options in the use of an organ, and so it’s question begging as to why we should either. One can’t appeal to nature and at the same time not notice how evolutionary nature actually works.
I realize this ungrounds ethics from objectivity of the sort we can bring to things like particle physics. But God has not given us a map for this territory (unfortunately). We don’t discover ethics “out there” like we discover electrons “out there.”
It’s why ethics should keep going back to love, not Thomistic essences. If the motive for actions can be linked to the better angels of our nature (love, curiosity, kindness, joy, etc.), that should transcend any claim as to what organs are “for.”
Thus gay marriage would seem to be an obvious example of how the Thomist strains out a gnat (how the sex organs function in reproduction) and swallows the camel (how big brain humans hijack evolution; it’s central to their nature to do so).
But if you’re moving toward love, you’re moving toward God (if God exists). To try to ground moral choices in nature’s nature, as opposed to God’s nature (love), is to run into problems with evolutionary hijacking. If you know evolution hijacks, then you can’t say what an organ is “for” in a universal context.
Let love rule.
Prometheus Unbound 