But (in my mind at least though I'm happy to listen to explanations as to why I am wrong) there are alternative ways of defining sex, which most of the time coincide, but don't always coincide. As a minimum, there's chromosomes, there's body appearance, and there's reproductive function. Of those, chromosomes are the most clearly binary and the most clearly unchangeable, and that seems to have become the standard definition. Given that that was not always the case, I'm just mildly curious as to how and when the choice to switch to that definition came about.