 Let's do a thought experiment. You're born as a male, grow up as a boy, identify as a man, you're a man, right? Then one day a test shows that without anatomical differences from other men you're one of the roughly one in 20,000 men born with typically female chromosomes. This, however rare, actually happens and for all I know it could be me. I've never taken a chromosome test, why would I? I'm a man! No matter your definition of biological sex you'll find these edge cases because nature is fussy and weird. Gender is not the same as sex, gender is something society enforces upon biology based on assumptions, sometimes wrong assumptions. Claiming gender is just simple biology is part of an often violent gender ideology that doesn't care about people or science. And if gender isn't simple biology then you have to ask yourself, what is it?