It's not "hijacking feminism" to insist on clarity about what a woman is.
Feminism is a women's liberation movement. Feminism recognises that women have been, and still are, oppressed and discriminated against due to our biological sex. Feminism is the belief that women should have equal rights and opportunities to men, and where this is not currently the case or is difficult to achieve due to the biological differences between us, women should have extra help and protection. Equality, but also equity.
This is precisely why we have our own single sex spaces and sporting categories: because society understands (or understood) that in order to participate in society on a level playing field with men, there are some instances where we must have separate accommodation.
Ignoring or minimising the very real biological differences between men and women, and choosing to define men and women according to "gender" instead, is the antithesis of feminism.
It's insulting, because it implies that sex based oppression is something that can be identified in or out of - that it is, in some sense, chosen.
It's regressive, because it defines women by reference to the very same sexist stereotypes that feminism was supposed to liberate women from.
And it's dangerous, because it undermines our ability to keep ourselves safe, by allowing any man who says he is a woman to come into our toilets, changing rooms, prisons and rape crisis groups.
How the hell is any of this compatible with feminism?
It's not.
Convincing liberal feminists that trans women - male people who have chosen to live their lives as women - are more oppressed than actual women, is one of the most audacious tricks the patriarchy has ever pulled.
We live a society where women are sexually harassed, raped, objectified, undermined, talked over, passed over for promotions, written off professionally because of our decision to have children or judged for our inability to have them or choice not to have them. The people who can take their makeup off, cut their hair short, put on a suit and tie and go back to being treated as men, with male privilege, are not more oppressed than the people who cannot do these things.
It's not difficult to see why so many young girls and women are looking at the way our society treats women and deciding they'd rather be a man. Some of them may alter their bodies to the extent that they can actually "pass" as men on a superficial level. But in order to do so they have to render themselves infertile, undergo invasive surgeries, take drugs which will have a lasting impact on their health, forego the possibility of ever really having a normal sex life or having children of their own. These are not sacrifices any man has ever had to make in order to live as and be treated like a man.
If all it takes for you to enjoy male privilege is to wear men's clothes, you are certainly NOT the most oppressed and vulnerable!
Feminism is for and about female people. If you make it about male people, and even worse, if you make it so that the small group of male people you have chosen to include take priority over the female people your movement is actually for, it no longer serves any useful purpose.