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