No, that’s a totally fair question!

This article summarizes some of the transphobic shit she has said. Here’s another one. She likes to say that her beliefs are feminist and meant to protect women, but really what she does is denigrate trans women and meanly argues that they can never be “real” women. It’s bigoted, hateful, and not what most people would expect from a “lefty” author.

You've already got answers with plenty to read, so I'll share an anecdote about why I feel so strongly.

She wrote an essay titled TERF wars in 2020 which has a few concrete bits I have beef with. TERF stands for "trans exclusionary radical feminism" and even besides the "trans exclusionary" part, I, a cis woman, have personally been harmed by radical feminists and their rhetoric. My intention here isn't to elevate my own suffering above that of trans people, who have certainly been harmed more by TERFs, but to highlight how hypocritical TERFs are, because ostensibly I'm one of the people they're trying to protect.

Doubly so because I'm also autistic, and one of the things Rowling says in her TERF wars essay was effectively "poor autistic women, being brainwashed into becoming trans men". It's really ick to be undermining autistic people's agency in this way, and it feels invalidating to my gender too, even as a cis person. I'm exactly the kind of person who Rowling talks about in her essay: an autistic woman who has a heckton of internalised misogyny to work through (made worse by the fact I work in science, so sometimes I do wish I were a man, before simulated gender dysphoria makes me realise that no, I actually just wish for a more equal world). I also have a bunch of LGBTQ friends, including trans friends, but rather than being peer pressured into being trans, if anything, I'm more secure in my own gender, which happens to be my assigned gender at birth.

This is because conversion therapy, in any direction, doesn't work. I don't recall if Rowling has said anything on this, but TERFs (the group she has specifically aligned herself with) are often pro conversion therapy.

Looking to TERFs more generally, a core part of their beliefs is that they define a woman as someone who belongs to the "female sex class" and that gender is a system of social norms that serve to oppress women. I deeply disagree with this because of how such sex essentialism serves to reinforce patriarchal violence. I have faced harassment from TERFs when going into bathrooms, including groping (they were leafletting and perceived me as trans and believed that my big boobs must be fake — a padded bra). I live in the UK, and I have directly seen how Rowling's TERFism has emboldened transphobes. They don't care who their "trans-spotting" hurts, which makes it abundantly clear that they don't care about protecting even cis-women, they just want to hurt trans people (especially trans women).

Trans women are the main target of their ire because of this same sex essentialism. They paint cis men out to be inherently violent or dangerous, as if having a penis makes one inherently predisposed to upholding the patriarchy. It is true that within the patriarchal system we live under, men are the privileged class, but I have countless male friends who recognise the harm done by the patriarchy to both themselves and the women in their lives. In my own experience of resisting patriarchal oppression, it's much easier to do that when you're working with men who recognise the privilege they have and work on the same team to undermine that oppressive system. I'm talking about something as simple as being at a meeting where I keep getting talked over by men, and one of the men, who is aware of how this keeps happening to women, speaks over the interrupters to say "hang on, Ann wasn't finished speaking. What was it you were saying?". They can do that much more effectively than I can, not least of all because they're less likely to face career harms for speaking up. Treating men as if they are fundamentally the enemy just reinforces systems that oppress women, as well as making the men who are aware of gendered oppression hate themselves as if they are guilty for the sins of all men. That's not productive.

What's real tragic is that in Rowling's TERF wars essay, it's clear that some of what drives her TERFism is her direct lived experience of patriarchal violence: in particular, her experience with an abusive male partner, and her inability to find justice following that experience. I truly feel for her, and sympathise with how that experience might facilitate the hostility that she now holds towards all she perceives as men. I don't doubt that she would feel unsafe at the prospect of a trans woman using a public bathroom, but I am literally scared of using public bathrooms because of transphobic harassment that I've faced.

TERFs ask "what is a woman?". Not me apparently. I don't menstruate (due to illness), so it hurts when Rowling tweets things that take the biological essentialism line that this makes me "not a real woman". And the tweets are where it all started. She said some stuff that wasn't especially bad, but some of her fans found them hurtful, so they said so. I've seen so many people try to constructively reply to Rowling, especially in the early days, but even handling her with kiddie gloves wasn't enough to stop her from blowing up defensively, deepening her bigoted rhetoric each time. That's part of why reaction against her is so strong, I think — many of her fans felt actively betrayed, me included. Part of Rowling's doubling down has been that she has surrounded herself with increasingly extreme people (like Posie Parker/Kellie Jay Keen, whose supporters include literal Nazis ) and continued to make it clear that she doesn't care about protecting women, only hurting trans people.

She's a gender critical feminist. But in a scale from 1 to 10 where 1 is a gender critical feminist that questions the validity of the concept of gender without ever wishing harm to people (actually trying to see what's helpful to all) and 10 is the raging transphobic lady that belittles and wishes harm to trans people, J. K. Rowling seemed to have moved from 2 to 6-7 (and she probably keeps moving).

This is a very controversial topic as many people think that all gender critical theorists or sympathizers are in the 10. I don't think so, but those who are transphobic are definitely louder.