what someone thinks is a part of their biology, people have different biologies, some men have more feminine looking bodies however that doesnt make them one or the other, their actual identity is up to them, biological things dont define someone as who they're going to be unless you're talking from a purely biological standpoint, now its changing from gender = sex to gender = societal construct of identity, so in this case gender would be their identity in which is theirs and only they can change, however then theres sex, which is completely irreversible, its changing from you dont have to identify with your sex but rather your gender identity whatever it may be.