If sex determined gender 99.8% of the time there wouldn't have to be entire social apparatuses that teach females and males how to be women and men. Our genders would simply unfold naturally from some preexisting biological sex. And as the entire academic disciplines of sociology, anthropology, and psychology demonstrate, that's just hooey. Humans are taught how to be girls and boys, women and men. Gender is a social construct that teaches humans to be women and men, part of which is the belief (unsupported by evidence) that biology dictates gender. If it did there would be no such thing as gender socialization. No parents, peers, or media schooling us in the "proper" behavior for little girls and little boys. No violence directed at those who broke the rules. No, gender is a lifelong social accomplishment, not a thing determined prior to birth by a person's biological sex.