Ignoring the huge fundamentalist religious support for transgender ideology cos it means they can trade their gay son for a straight daughter, see Iran for example, but also large chunks of the US.
But it does kinda suggest God makes mistakes, which a lot of religious people aren’t keen on.
Same reason a lot of crunchy types who don’t believe in big farmer (I know, it’s intentional), are anti trans because “you shouldn’t outsource your happiness to drug companies/surgery” which is basically an atheist “God made you perfect who are you to argue” type thing.
As an ideology it steps on a lot of toes that being gay didn’t. Religion (like the gays), social conservatives (like the gays), but also rationalists, second wave feminists, material socialists, anti-medicine types, conspiracy theorists, “leave the kids alone” mums groups.
Lots of groups of angry people with strongly held metaphysical beliefs.
Feel for the actual people caught in the middle of such a huge metaphysical debate. They’ve been done dirty by activists pushing past rights and into science, biology, neuroscience, etc. I get the wish to have something solid to base your rights claims on, but I think it’s a mistake as then your rights are subject to the evidence which can be debated.
Gay rights weren’t brought in because science said so or we thought gay people had an untreatable condition we should make allowances for, if they were the same arguments that are still unsettled today about “why” someone is gay would be getting in the way. They made the pure rights point of “what does it matter to you if I’m gay?” And won the vast majority over. No need to drag the pendants in and start combing over genes or get into Twitter fights with Richard Dawkins. Just need to take on the “marriage is a man and woman” crowd which is comparatively tiny and socially seen as a bit weird anyway.