I think the term you are looking for is an "appeal to nature" which differs from the naturalistic fallacy. My comment was not appealing to nature, in that it made no judgement - good OR bad - in defining the synthesis and self-infusion of hormones as unnatural. It just is unnatural. By definition. In contrast with the OP's comment, I believe that's something we should all be able to accept without strife.

While it's true that everything we do is literally a part of nature (nature being the world around us), it makes sense to have a word to define biological substances or events that wouldn't otherwise occur without human intervention. E.g. artificial colors, artificial flavoring.

But why is it important to make the distinction

We don't have a distinction for when other animals do things, there's no logical reason to separate humans

We do have adjectives to distinguish things that other animals do. When a baby sucks on a teat it is mammalian. When turtle dives underwater then comes out on land to bathe, it is amphibious. When something takes flight it is avian.

Humans are unequivocally more advanced than other animals, so to represent those actions which wouldn't otherwise occur without human presence, we use the term "unnatural". It's a nice adjective - albeit in the context of transgender modification "unnatural" serves no scientific basis, it is just a descriptor, if that's what you're getting at.