Besides, "there are only two sexes" is rather obviously inaccurate. While intersex people aren't terribly common, they do exist and are well-documented – as are the genetic reasons for why they're intersex. XX men and XY women are also a thing. Genetics are inherently messy.

But acknowledging all that would mean having to admit that sex is a complex matter and can't be handled with simple statements like "the one you were born with is the one you should have". It's easier to just pretend intersex people don't exist.