Comment by hermitcrab

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.