MissBax that is a good question.
I think personally it has been the main gain of the transactivist lobby, to switch the discussion from transsexual to transgender, because everything else follows from that linguistic sleight of hand.
If we talk about gender, we are talking about external appearance and social norms, not biology, as many others have pointed out.
Thus, if we talk about gender as if it were sex, we can believe the lie that changing gender= changing sex. This involves forgetting, however, that gender is a social construct which changes across time and place, and is really a bit meaningless other than an accepted way to organise society hierarchically. The hierarchy of gender is historically based on reproductive biology, but it is not determined by it.
So second wave feminists argued that gender roles should be deconstructed and women should be able to have the same opportunities as men, regardless of biological sex. (They were gender critical in today’s terms, but used the term gender distinct from sex, to delineate social from biological; they never argued biological did not matter, that is why we have access to contraception, abortion, DV refuges etc)
Third wave, neoliberal feminists argued that gender was performative and that performing it was a choice and could be empowering. This meshed with neoliberal capitalism to consumerise gender choices.
Fourth wave feminists argue that gender can be performed by anyone regardless of sex and if a man says he is a woman, he is. I do think people here genuinely think expanding the boundaries of what it means to be a woman is in line with feminist thinking, and anything else is discrimination.
Add developments in surgery, synthetic hormones, a whole new market for make up and fake boobs, and a way for men to access women’s safe spaces and undermine their hard-fought gains, and bingo, it makes commercial and political sense to talk about gender and not sex.
Because if you cannot change sex, then there is no reason for men to take hormones, have surgery, buy make up, enter women’s spaces, and keep the neoliberal capitalist economy and patriarchal complex upheld.
Whereas if we talk about splitting bathrooms by gender, we are really talking about external appearances; people who look like women (very superficially) and men who identify as women (by adopting the superficial, stereotypical trappings of femininity) can go into the women’s bathrooms, but also keep the whole pharmaceutical, capitalist, patriarchal complex going - and people can seem right on and liberal and applaud it too.
Not sure if that answers the question, though. But I think if it is possible to answer why we all need to talk about gender, not sex, then we are half-way to understanding what this is really about.
Because you cannot say on one hand there are multiple genders, and then try to shoehorn them into two bathroom categories. That is a nonsense.