So it used to be "transsexual," but "trans folk" knew that the general public would never buy that. Probably the first sex-related lesson every human throughout history has learned is "boys have a penis girls have a vagina." It just wouldn't work. So "trans folk" decided to go about it with a bait-and-switch by:
1. Separating Sex and Gender. Yes, sex is immutable, the activists said, but *gender* is a social construct. You identify with the gender that matches your sex, but I don't! My brain isn't a male brain, but a female brain (they still use this even though the science has completely failed to support it, ever). Because if gender isn't biological, then who are you to tell me what my gender is??
2. Replacing "sex" in all of their jargon with gender. Thus "sex reassignment surgery" became "gender confirmation surgery" and "transsexual" itself became "transgender." The condition of *gender* dysphoria became the north star of the movement, providing a clinical, science-based diagnosis to validate their claims (note: although debate remains, many [most?] trans folk have abandoned the idea that you need to have gender dysphoria at all now). Now with the backing of both sociologists and psychologists, to question trans folk on the subject of gender became rote bigotry.
3. Subtly redefining how everyone else experiences their own *gender.* What we used to call gender norms or gender roles now becomes "gender identity," and even though you've never thought of it along the lines that the trans folk are drawing, you must nevertheless comply, or risk "invalidating" the trans folk, and being labeled transphobic. You liked G.I. Joe action figures as a child? Of course, you're a boy! But your sister also liked G.I. Joes? Sorry, but she's not the tomboy you all thought she was, she was actually a boy too, and to refuse her (dangerous) medication was literal bigotry. She says she's a girl? Oh, the poor soul doesn't realize it yet, such is the sorry nature of internal transphobia. Make sure you tell her that every emotional problem she experiences during puberty is a direct result of her being "born in the wrong body," and when she's ready to accept that, send her to us.
We're currently in step 4 of this convoluted maze. The final turn will have trans folk telling us that gender does indeed equal sex---to deny such a reality is harmful, transphobic, and in some places legally actionable. Yes, gender does equal sex, so my penis is really a female penis, his vagina is a man's vagina, and homosexuality is inherently bigoted for refusing trans folk their human right to be in whomever's dating pool that they wish. You may be a gay man, but if you work on it you can overcome your limitations! You can educate yourself on my male vagina, and together we can have wonderful gay sex, which is to say penis-in-vagina sex! Is there any gay sex that's gayer than heterosexual sex? NO! trans folk say, And we'll get you fired for asking that question!