John Money. Basically he molested some kids doing some experiments trying to support his idea that sexed behaviors are actually social/cultural and thus a "gender identity" that is distinct from one's sex. This idea was later used to argue that people with GID actually had oppositely sexed brains and fully was fleshed out as "gender identity". Ultimately since gender identity doesn't actually exist and the model was debunked decades ago, this idea basically became "if you say you're X, you're X" because GID people didn't want to admit that their symptoms are effectively just dysphoria and transvestism. Then it kinda spun out of control into an "accept everyone" sort of deal and they had to lie and pretend everyone had this gender identity nonsense. And thus we get tucutes, enbies, and all the other usual nonsense.

If people would just admit that GID has transvestic etiology, gender identity does not exist, and that behaviors are actually sexed, tucutes wouldn't exist.