Hopefully I'm preaching to the choir and the main response from everybody is that this is obvious and didn't need to be said.
I read a few comments from some gross cishets about how angry they are about the female Custode. They prop up the Imperium of Man as though they are some paragon of morality and not the picture of everything wrong with imperialism. Naturally this perfectly moral force would have only men in its ranks, clearly! They don't seem to realize that if women are left out we have a sexist/mysogynist system which is not worth praising, and if they include women then it's the imperialist machine trying to keep up appearances. Frankly, I've always figured the Imperium didn't care about gender or race because everyone is suitable to die in combat and be ground into food for the starving populace.
tl;dr Men are upset that women exist and missed the whole satire
My headcanon was always that there have always been female Astartes, but they're just so roided up from a young age, it's impossible to tell a male Space Marine from a female one without seeing their genitals, so they all just present male and refer to each other as brothers. They've also been so indoctrinated in their cultiness from a young age that they don't even have a concept of gender identity. They consider their genitals to be a personal matter you just don't talk about.