It’s not really about physical characteristics that match the letter as much as having another identity marker to differentiate people who may have the same name, and to track them across systems. We could theoretically give everyone a random letter of the alphabet and probably be just as good. We’ve had gender nonconformity for ages. In the Middle East, men without beards are feminine. In the 60s US, men started wearing their hair long like women. This is just an extension for that