I thank you for demonstrating the Catholic sanctimony in question by insinuating repeatedly that I don't understand the basic issues of Aristotle's teleology.
This article links to a long article of mine on Edward Feser's Thomism that goes into Aristotle's metaphysics. See also my article on Cosmic Skeptic's podcast with Feser (link below). When I deal with something at length elsewhere, I don't need to repeat myself. There's also no need to go into such metaphysical details when dealing with the Catechism's natural moral law theory since there are basic objections to discuss, such as the naturalistic fallacy and the theory's conflict with the original sin doctrine.
Suppose it's true that the function of sex is to procreate. Does it follow logically that we morally ought to have sex only to procreate? No, because dysfunctionality is possible and you'd need to equate that with immorality. On the contrary, we may be obliged to outgrow our natural functions, especially if those functions evolved when we were only proto-people, hundreds of thousands of years ago. You see, it matters whether the function evolves in the wild or is assigned by a perfect deity. The moral commandment to act functionally would be hidden in the presupposed theism.
Alternatively, if dysfunctionality is impossible, and we can't possibly go against our formal essence, then morality wouldn't be at issue since morality deals only free beings who can make choices. If you can't choose to violate your essence, there's no moral merit in being enslaved to that metaphysical necessity.
And again, if there is such a choice--as there obviously is--you'd have to show that we're morally obliged to defer to our natural function. If our function is to be free and creative, that's going to support so-called dysfunctional explorations, such as the homosexual's.
You can attempt to derive an "ought" from an "is" if you life, and you may be successful if your argument works. But Aristotle's teleology deals with virtues and vices, not with the kind of strong moral obligations found in religion or in Kant's duty-based approach. Aristotle shows there are natural functions, and Darwin explained them in updated, scientific terms. Those functions aren't the same as moral obligations, as I showed, and as you've ignored. Biological functions are for animals and organs, not for autonomous people. Moreover, this interest in reductionistic philosophy conflicts all over the place with Christianity's emphasis on human freedom and Platonic dualism (nature vs supernature).
