Biological sex isn't defined by beards or breasts.
It's defined by the reproductive roll of an animals gametes. They apply through all sexualy reproducing species on earth.
You can't in one place, say sex and gender are unrelated; Then another place use them as synonyms.
I'm all in favor of trans rights. And I believe seperating sex and gender is great! But it needs to be consistent. This inconsistent selective misuse of terms is infuriating. And I think it's a big part of the hangups otherwise liberal people have with this movement.