If you’re a Thomistic-style theorist of natural law, you look at an organ like the penis and say that, obviously, its form and function are directed to reproduction, and thus, if you put a condom on it, you are (and this is in the words of an actual advocate of natural law) “positively frustrating a natural faculty.”
Poor, frustrated penis.
But what about the poor, frustrated human imagination? In other words, evolution has not just acted on the penis, giving it its form and function, but on the human brain, giving it its form and function.
And that function is to exercise imaginative routes around the natural course of things. That’s the human superpower; not to act on instinct–on the given–but to imagine alternative futures. Thus does Thomistic natural law drop the context of the human organism as a whole, setting the evolved organ of the brain against the evolved organs of reproduction. Like the libertarian ideologue who only focuses on freedom, but never equality, the natural law theorist doesn’t address the balancing of competing goods–of competing organs.
Thus a man who masturbates does this so as not to frustrate his faculty for fantasy–his brain. In masturbation he “positively frustrates” his penis’s reproductive function, to be sure, but it pleases and fulfills both the brain and penis in other ways.
Likewise, a woman may regulate her fertility with contraception so as not to frustrate her graduate school education–the brain’s desire for knowledge. She makes a decision about competing goods pursued by competing organs (brain and reproductive organs) that cannot be proscribed in advance.
The environmental context of the organism is also important. Rural agricultural life (such as that lived by an Amish hausfrau) and city life (such as that lived by a single woman in NYC) entails a different weighting of competing goods (how much time devoted to education, how much to child-bearing, etc.).
So the nostalgia here is that Thomistic natural law can provide substantial (non-trivial) guidance to both the rural and city woman by reading off the function of their sex organs in decontextualized isolation.
Thus the virgin Thomas Aquinas’s notions about women and sex belong to a pre-Darwinian agricultural era when females were married off at fifteen, and when advanced education and the holding off of marriage for young women was unthinkable.
Life expectancy, after all, was under forty. It was a different world.
Now it’s eighty. And demographers tell us that 90% of all human beings on the planet will live in cities by the end of the 21st century. The concrete jungle does not support the sexual mores being advocated by contemporary natural law Thomists, and where they’re seriously tried there it leads to ludicrous ordeals for married women (such as attempting to regulate fertility by the rhythm method).
So if you want to talk about frustration, think of the time-consuming monitoring, mental distraction, and aggravation of married women practicing the rhythm method–which largely doesn’t work anyway.
Thomistic natural law theorizing thus sets at war brain and body, city and countryside. It is a way of thinking about humans that is sympatico with special creation and authoritarianism, but not evolution by variation and the democratic exploiting of contingencies.