Thanks for this.

Yes, I think there's fear partly rooted in general resistance to medical and scientific progress. From my sense of the world (though I don't have numbers on this), denialism of climate change & conspiracy theories about vaccines correlate with hostile "skepticism" toward trans people.

Last year, Jennifer Bilek tried to coin the term "Synthetic Sex Identities" as a replacement for "transgender," claiming that "wealthy elites" are funding gender-affirming care, succumbing to their "human temptation to play God.” It's overt transphobia drawing from antisemitic tropes about tiny cabals controlling the world, in this case blaming the doctors more so than the patients. (I also wrote an article about Bilek.) This one explicitly is fear of transhumanism, as you suggested.

In the future, I'll keep an eye out for when GCs simply say that sex is real or when they say that gender is important because sex is real. GCs try to have it both ways on whether gender is important. How it works in practice is: Whenever they have thoughts, feelings, observations, opinions, etc. about gender, they understand themselves to be describing sexist oppression they've encountered in the world as it is and discussing it in an open-minded and nuanced way with feminist goals of how the world should be, but whenever trans people share our thoughts, they understand us to be part of a sexist delusion or conspiracy to impose patriarchal gender stereotypes illegitimately and offensively on everyone else. Their fallacy is their suspicion of other people's ideas on the grounds that they don't know what other people's bodies are, while remaining confident that their own ideas are correct because they know their own bodies are correct. I think there is often a "because realness" embedded there, so it's the naturalistic fallacy, as you say. Thanks.