Okay, a little disingenuous maybe. Some people would probably still class that with the other things involving reproduction. Nevertheless, I had some thoughts.
The Church is obviously pro-breastfeeding. Not that it's required or anything if it can't be done, but there is a definite encouragement or preference for it as healthy for the baby and mother physically and emotionally. Obviously, weird fetishistic things wouldn't be approved of, but I'm sure for good reason even an adult could breastfeed or take breast-milk, especially if it were the only nourishment available in an emergency (ala the end of The Grapes of Wrath).
However, I had a bizarre thought about a world where hordes of women were lined up in factories producing breast-milk industrially for sale to the general public for some type of fad.
Something about this seems...unwholesome. In fact, it seems disgustingly decadent to me. And yet, what moral principle would it violate, exactly?
At first I was inclined to see it as a form of prostitution, an alienation and commodification of the body for material gain. And yet, then, I realized...the Church has never had any sort of formal teaching against wet-nursing (where one woman pays another to breastfeed her child). From what little information I could find, I saw sources claiming both that the Church long inveighed against wet-nursing, but then other sources claiming that the Church actually promoted it as a way to allow the wife to more quickly return to her "conjugal duties."
And, of course, any labor for pay (at least in our current capitalist system) could be seen as an alienation and commodification of oneself. Marx pointed this out. It's one reason why I think people making a big self-righteous moral differentiation between our economy and ones that have "official" slavery or serfdom...are making a much bigger distinction than actually exists in practice.
Our economic system still alienates and commodifies people according to a certain structure of labor and class, is still totally exploitationist. Approving of this (or at least tolerating it morally) while getting all high and mighty about societies with a more rigid class structure that may have included slaves or serfs...strikes me as rather delusional. Having to work by the sweat of our brow for our keep is simply one of the results of the Fall.
So Catholic doctrine doesn't condemn absolutely any such arrangements of labor or economic class structures in themselves (not even slavery in theory), because it knows that the distinctions between them are in many ways artificial; the difference is in degree, not nature. Though it always had condemned abuses and indicated certain principles pointing to better alternatives.
And yet, something still seems really wrong about the industrial breast-milk idea. Even though we drink other animals' milk. Even though there is a legitimate nutritive aspect there. Even though people alienate and sell the products of their hands all the time. Even though I don't feel there is anything wrong about selling, say, human hair for wigs.
Maybe I'm just being squeamish? Or maybe this hypothetical is illustrative (through being extreme) of everything that's wrong with economics in general which, while maybe we can't condemn it absolutely, certainly is not ideal for human dignity.