Comment by hermitcrab 4 hours ago
Not a biologist, but my understanding is that gender isn't that simple. As well as XX and XY, you also have XYY, XXY, XXX, intersex, hermaphroditism, chimeraism etc (the last 2 are very rare in humans).
Intersex is not ultra rare. About 1.7% of people are born intersex and with intersex babies it is sometimes hard to tell what sex they are from the genitals.
https://www.ohchr.org/en/sexual-orientation-and-gender-ident...
Also you can have people who develop to look like completely normal adult females, but are actually biologically male.
https://medlineplus.gov/ency/article/001669.htm
It's complicated.
Why don't we use the definition based on genitals, that a doctor uses to pronounce you at birth, and not the ultra-rare genetic exceptions?