You can't ban adult idiots wanting to castrate themselves.

I don't see why not. We ban adults from doing a lot of things.

Now sure I understand not making it illegal for someone to literally castrate themselves - but that's not what's happening here. Make it illegal for doctors to remove perfectly functional body parts of a patient. It's a barbaric practice. Give prison time to anyone who performs, assists, solicits, or funds such a procedure. (which includes the patient)