two words: intersex people
They’re born with parts that can be neither male nor female (no, I don’t know what they look like, that’s their business, not mine. Also I don’t want genitalia in my search history) or they can have a p**is and a va**na at the same time (again, I’m not looking at random genital pics)
And biology classes (excluding college ones, but especially high school ones) have been known by professional biologists to be so compacted and so rushed that it’s to the point of inaccuracy. They don’t get the time to explain everything in full detail until college classes
And sex is different from gender identity (has been proven by studying the neurological patterns in trans binary and non-binary people)