That wouldn’t work. Conservatives needed to write iron-clad laws that no trans person could find a way around. If you base it on genitals, what happens when someone goes and gets sex reassignment surgery? That’s not done on minors, but conservatives don’t know that. How’s a school coach to know that he isn’t a trans guy that snuck off to Thailand one summer and got a dick installed? This is why these laws reference the original birth certificate. The entire point is to make it so sex is immutable and unchangeable.