Why do people use human abnormalities to prove sex isn’t binary?

Desperation.

The problem is in your last sentence: it's not rational.

We try to teach our children to be inclusive of people who in any way look different to them, not to be racist or gingerist, for example. We recognise that people born with, say, additional or missing fingers, are neither to be mocked not to be forced to conform to the norm. We recognise that people with disabilities are equally valid to people without disabilities, and that understanding how to support people with disabilities is a far better way of creating social equality than pretending that either they or their disabilities do not exist.

Some people have swept it all together and made acceptance of difference into a neo-religion which is far more important than functioning in the real world. This attitude IMO is just as harmful as the earlier belief that people with differences should be reshaped into something that looks acceptable to society, or else hidden from society.

Because it allows the gullible to believe that people aren’t necessarily what they appear to be. Wolves in sheep’s clothing really are sheep, you know. The plump bald man behind the curtain really is a great wizard.

Your horse is a unicorn and I’m the queen of the faeries.

And along with that comes the tantalising opportunity to be free of all responsibility for consequences.

I think it's all they've got. If your political goals depended on proving 2+2 = 5. The first thing to do is cast a sliver of doubt on the idea that 2+2=4.

You'd say maths is complicated, and waffle on at length on topics like complex numbers in the hope of bamboozling your opponent.

You cast yourself as the learned professor teacher trying to convey nuance to the hopeless uneducated simpletons, who still havent progrssed past 2 + 2 = 4.

Firstly because the fetish side of the whole movement was very well known about and providing an “evidenced” medical reason garnered more sympathy, support and encouragement to push for certain laws.

Secondly claiming it is biological means there are children who are born trans, thus ensuring more likelihood of parents and society getting on board. Trans children also provide more “authenticity” to adult transness.

Separately, I see it as a modern version of a Victorian curiosity show.

it demonstrates really poor understanding of mammal or specifically human reproduction.

But it also relies on really poor understanding of basic science to fool others into thinking ppl who say they’re trans are due to biological factors.

it’s a Trojan horse.