Okay, I read the original post, which was long and annoying and unconvincing. In fact, it was hard to extract what she was trying to say. But let me try to break it down.
She said she doesn't want transwomen to use women's bathrooms because men might impersonate transwomen to get in there for malign purposes. That sounds highly unlikely.
She also doesn't want men to pose as transwomen to get the benefit of charities for women. Again, does that ever happen? No. To me, it sounds like the KKK claim that they had to lynch Black men because otherwise they would rape white women. But that never happened! It was just an excuse to harm and kill and Black men. In fact, in a diabolic twist, it was white men who were raping Black women all along, as is well documented. Ugly and upsetting.
Finally, she says she doesn't want children to be able to transition easily, because they might regret doing so later on. This is similar to the patronizing argument that women should not be allowed to have abortions because they don't know what's best for them. Honestly, no one will know better than a child's parents what is best for that child. Certainly not a celebrity author. It's not JKR's business, and butting in is causing real harm to real people. So she should butt out.
Oh, yeah. And she doesn't like to be harassed on Twitter. OK. Fair point. Threats of violence and namecalling are never appropriate. But overall, I agree with Sady Doyle that JKR shouldn't be using her fame to rally people against trans rights.
Why can't we just all get along?