She is quote notably anti-men in many respects. It shows in her work quite a bit, and much of her clarification about plot points alludes to it. This, naturally, extends to her views on Trans women.

It's important to note that she has a history of being sexually abused. I won't go into the details here, and it in no way justifies her stances. But it is important to recognize that her views didn't just appear out of the ether. There is nasty history, there.

She has very apparent qualms with the quality of male character, by nature. When describing *spoilers for Harry Potter (whole series) ahead* why James' death didn't grant lily or Harry any protection, despite facing Voldemort unarmed just like Lily did, she went on record sayingthat James' sacrifice was more akin to "animal instinct" than real love; that there is nothing even comparable to what a mother has. This is reflected further in the series, ad the further we get in the books, the more James is trashedand forgotten, and the more Lily is practically deified by everyone who talks about her.

What's more every male character gets steadily less reliable and more flawed, while her female characters... don't. Ron gets more jealous, angry, and distant, while Hermione's biggest struggle remains "being so good at everything that she's afraid to fail." McGonoggal is a near-perfect role model, while every male teacher systematically fails. In the last book, Even Lupin suddenly has some totally-out-of-character "crisis" where he tries to run out on his pregnant wife for absolutely no reason.Sirius gets ripped down as a terrible slave-abuser and bully right before and after his death. Dumbledore gets a ton of time dedicated to how secretly awful he was.

While it's not universal (Hagrid, notably), her male characters are notably much more flawed and unreliable than their female counterparts. they're mentally weaker, more temperamental, and whenever there is a conflict between the two, the guy is the one flying off the handle and in the wrong. TBH it's a miracle that Harry ended up being the protagonist at all.

I dont say any of this to try to diminish feminism, or say "durr, how DARE she make the men worse than the women!!!1!" That's not the point. All I'm saying is that her writing TENDS to skew in portraying male authority figures in a worse light; which makes sense, given her history.

That said, those tendencies spill into her view on Trans issues. Because she views Trans women as basically "men playing dress up," she assumes that absolute worst case. She assumes that same level of flaw, weakness, temperamentalism, and "animal instinct." She sees trans women as predator sleeper agents, because that's the only reason a "man" would want to be a woman.

It's incredibly fucked up, but her stances have been there from the start, for anyone who cares to look. She has a very low opinion of men, and as far as she's concerned, trans women are men.