Your question misses the point entirely.
What hasn't quite sunk in is that defining gender solely by the genitals someone is born with is itself arbitrary and culturally specific.
That is not, and never has been, a universal definition of gender. It's not how India has defined gender for at last 2,000 years, nor how most Native American tribes defined theirs. It's not how the Buginese (with their 3 sexes and 5 genders) define gender. And it's not how ancient Egyptians defined gender (they had 3).
It's not even the definition of gender in your own culture.