J.K. Rowling

It's a cult. She's a fascist and both biological and gender essentialism is useful to fascism, capitalism, tyranny and authoritarianism as it allows people to be more easily controlled. Whereas if we actually saw both sex and gender as what they are: spectrums, social constructs and ever changing along both life and through our understandings then we'd be a lot more free and those people would have less power even if, ironically, in the end they'd end up with less in the current or a fully fascist system too. They hate having less power in certain ways so they have to keep it around. That's all.