It's not like "gender is determined by your sex" is all she said. The comment about bathrooms IMO caused the biggest outrage.
Here's another one from the UK that might be a bit more relevant as she's from the UK. Plurality for "a transgender woman is a woman", and for "transgender women should be able to use women's changing rooms". Both are vehemently opposed by JK Rowling. The demos that do agree with her are older Conservative men (who are not exactly a strong core of the HP fanbase).