This is more of a meta thing, but relevant to a lot of comments I’m seeing here. Having an opinion about pineapple on pizza is the most uninteresting cultural phenomenon. I’ve spent the last 4 years on dating apps, and at least 1 in 3 people write in their bio about this “issue”. It’s not something that people truly have strong feelings about, it’s like straight men saying Ryan Reynolds is attractive, or people arguing over the definition of a sandwich. It’s an opinion that people hold as a proxy for being somebody with strong opinions.
I’ll put my copy-paste argument here:
She believes trans women aren’t women. She believes trans women are only lying in order to get close to women and rape them. JK Rowling self identifies as a a TERF, or “Trans Exclusionary Radical Feminist”.
Rowling donates money to the LGB alliance a UK “charity” that actively lobbies against trans rights, and has fought for the continued legality of trans conversion therapy.
Rowling has tweeted out in support of one Maya Forstater, a woman who was fired and has become a martyr for intentionally misgendering trans people, and once again perpetuating the idea that trans men are rapists. Rowling keeps pushing the idea and propping up those who agree with her.
Rowling has posted to advertise for an online shop that sells, among other things, anti-trans merch. The store is ran by an Angela C. Wild, a founding member of “Get The L Out”, an organization that has in its mission statement “We stand against any kind of misogynistic politics and systems that prioritise men’s interests: queer politics and transgenderism, right-wing politics, left-wing politics” etc.
Under the pen name Robert Galbraith (a name coincidentally shared by a 20th century proponent for conversion therapy), Rowling had published multiple transphobic books, writing one book about a serial killer who dresses as a woman to lure in unsuspecting victims, and another with a self insert character who is killed after “speaking out” against transsexuality.
This is just the tip of the iceberg. There’s a great video that goes into much more detail than I have here about the causes she supports and the right-wing activists and think tanks she associates with and gives money to.