Or maybe better to say, are not the sex they appear.
But there is a large group, and I think this is very very common in places like Canada, who really believe this is a scientific thing.
Your whole post makes a lot of sense. I've grabbed these bits because I realise that they describe where I was before I started looking in to all of it with more scrutiny.
I was aware that I was forcing my brain to make the leap in the first sentence but I was happy to do it because a) it was kind and I didn't think it affected anyone else (obviously I know different now) b) of the second part.
I had seen a documentary ages ago which left me thinking that a trans person's brain was physically closer to the brain of someone of the opposite sex. I can't quite remember the details now but 4 brains of gay people (the documentary muddled up trans and gay) who dedicated their bodies to science were examined and there were some parts of the brain that apparently proved something. The only other things I remember about it is that they also spoke about David Reimer, how he knew he was a boy, and (separately) the brains of psychopaths.
It convinced me at the time that there was a scientific difference.
I'd love to see it again now and see if my takeaway was different. I very much think it would be.
For example, the only science I would now consider to be linked to the David Reimer case is to do with testosterone. We know that testosterone is linked to aggression and libido (Helena and Sinead, the detransitioners, talk about their experiences of this) and we know that boys get testosterone in the womb as well as at puberty. David's body would have had his first dose of testosterone in the womb but I suspect most of David's childhood unease when he was "living as a girl" was related to the years of sexual abuse e.g. the sexualised photos that he and his twin brother had to pose for. Presumably his unease and likely trauma-influenced behaviour, due to the abuse, led his parents to think that the problem lay in the fact that he "knew" he was a boy.
But I now fully understand that I was wrong in what I thought to be scientifically correct. This would have described me...
Very few people will be able to talk in a knowledgeable way about genetics, black holes, or cell division. So they aren't all that put out that they can't explain gender and sex.
... and the first part still does. My scientific understanding is still limited but I know enough to be confident to say that the amount of testosterone in someone's body varies - there is a male range and a female range - but this does not link in any way to a male gender identity and a female gender identity. Instead, it's probably what's behind the fact that the vast majority of violent and sex crime is done by men (aggression and libido). A man with lots of testosterone isn't "more of a man" than a man with less. Also it's not the only hormone responsible for mood, so violence and horniness aren't solely linked to this. Plus of course it's perfectly possible to have levels of testosterone in the male range and not commit violent and/or sex crimes. However, the animal world shows us what it can look like when there aren't laws to stop males from acting on these urges. Our laws have influenced our culture, which influences how men and women behave.