So apparently J.K. Rowling went from feminist idol to TERF this week.
While I myself am very vocally anti-sexist I must admit as a community the majority of feminists will throw you under a bus the second you step one ideological toe out of line. She backed that horse through much of her modern career trying to pander to the feminist community to increase her fame in insincere and largely meaningless gestures. She made her bed and now she has to lie in it.
Personally this is why I am so blunt and willing to discuss or consider unpopular ideological views in any group. I know this will be the outcome no matter how hard I try so might as well be intellectually sincere in my public evaluations of the world.
With that said I'm not clear on J.K.'s actual opinion beyond a few vague tweets so I cant say if the attacks have any justification.
She does seem to think sex is immutable, strictly speaking she is right, it is the genders that trans-people change, not their sex. But the ore important aspect is that unless you are a medical doctor talking about someones genitalia then any use of pronouns you choose or the word "male" and "female" are and have historically been references to gender not sex.
So while her statement strictly speaking was a bit of a non-point, it does suggest she is rather clueless as to the linguistic and underlying considerations of the discussion.