The prefix is about men because our society has deemed it masculine. If a girl sits with her legs apart she's told to stop because she is sitting like a man. If a man sits with his legs crossed he will be shamed for being to feminine. Femininity and Masculinity are social constructs made to force men and women to act in favour of certain stereotypes.
Men get hurt by sexism too. Feminism is equality for men and women. A man should be able to do what a woman can and vice-versa. This is what feminism promotes, not a women are better ideology.
You know if we're going to use "man" prefix like "mansplaining", "manspreading", "manthreading" and so on, then we should be able to use "fem" as a prefix as well like "femsplaining", "femspreading", "femthreading" and shit like that. It's only fair. And isn't feminism about equality and fairness?
But not THAT kind of equality!!1!