It's the impossible trifecta.
- We have to separate by biological sex because vagina owners have to be protected from penis owners.
- But we can't like... check for penises and vaginas at the door because that's just... all kinds of horrible.
- But we also can't go "Okay penis and vagina owners you have to do X (dress a certain way, act a certain way, whatever) so we know if you're a penis or vagina owner without checking what's in your pants."
And that's where we are stuck. All 3 of those can't be implemented in the functional real world at the same time.
The debate is which side of the triangle we get rid of. One of them has to go (or be heavily modified or deemphasized depending on how much of that hair has to be split.)
And you have to accept that, the impossibility of the whole triangle, even if you don't fully disagree with any side of it. There's arguments, logical sane arguments, for each side but they all 3 can't be "real world application" true at the same time. We can't make this not true.
And again I can lay out what the actual problem is without saying sex, gender, male, female, cis, or trans.