If actual brain sex were real that would legitimise transgenderism because people's bodies actually could be misaligned with their brain

No, because the brain scans have shown trans people have normal sexually developed brains for their sex and SO. What's up with them is a bit that handles body perception, and that's also linked to OCD and a bunch of other mental health problems. So.. mental health issue not, ' wrong brain sex'.

In order to have body brain sex mismatch you'd need totally the wrong prenatal hormones, and that would leave you with intersex genital development. So the outside would still match the inside. You'll see this in severe cases of male androgen insensitivity, and in girls with severe CAH (excess prenatal testosterone). CAIS males have typically female brain development.

About the only time you can get even close to this is in DHT insufficiency, where you'll get a normal male brain in what looks like a female body, like Caster Semenya. Which only lasts until puberty when the normal testosterone causes male muscle growth, voice breaking and size and you get an obvious male. With a vagina and internal balls.

And the little boys with DHT insufficiency normally ID as boys long before puberty kicks in, wanting male playmates and avoiding 'girly' toys and clothes. Which does strongly suggest gendered behaviour has a hard ware component in humans. Because there's no real way to tell what they without a medical scan. This isn't down to socialisation. Cordelia Fine avoids discussing this group in her book for a very good reason, it shows a lot of behaviour is hard wired. And she also avoids the fetal testosterone experiments for the same reason.

Humans that are homosexual have brains that fall between the two norms. They also have other behaviour between the sex norms. For example lesbians have higher offending rates, which does cast doubt that male offending is down to socialisation and adult male testosterone. You see higher rates of aggression and male play behaviour in female mammal embryos that you dose with testosterone and make gay.

There's a reason for the stereo type of gay males being effeminate and lesbians being butch. They've had intermediate hormone exposure as embryos. It affects speech, body language, play, facial development, offending, body language.. which is probably why AGP TIMs don't ever pass but why some HSTS do.

Humans are just animals, we're not special.