Biological sex can be determined yes, and there are rare "intersex" cases too. But social gender roles are a different thing. Some people think that they should have other sex's traditional social role, but it doesn't make sense to me honestly. If you want to fight against sex-determined stereotypes(i.e. gender) that don't apply to you then you must fight against the whole notion of gender rather than trying to adopt a different sex's set of stereotypes. So this stereotype of "gender" isn't a fact but rather ideological opinion which people can have a different opinion on.