What do you mean by "sex"? Would a woman who goes through menopause no longer be a woman because she's not fertile? What about someone born intersex with multiple or no recognizable genitals? What about a woman with a Y Chromosome? That's not hypothetical it's recognized by actual biologists. The reason it's under diagnosed is because it rarely shows symptoms so it's only something someone already taking a DNA test would find out.
Fun fact though, when you meet someone, you forgo the strip search, DNA test, and ultrasound required to actually verify these things. Not only is gender not based on sex, YOU don't base your evaluation of it on sex. Instead you've picked a number of unrelated arbitrary qualities, which is what scientists and I call "dumb". If we're assigning genders willy nilly why should you be in charge of everyone? Let everyone pick for themselves.
Let me ask you a question to clarify things: Is ANY gender based in ANY way on the sexual organs (and thus the biologically assigned sex) of ANY human?
That's okay. I'm peering through the internet and into your mind.
> Is ANY gender based in ANY way on the sexual organs (and thus the biologically assigned sex) of ANY human?
Nope. See my above statements for proof.
At what point did everyone stop strip searching people and inspecting their reproductive organs? Must have been before I was born.