Ah, so you misunderstood the post then, I think.
The post was asking about thoughts or disagreements about the idea of the Y chromosome's "short leg" allowed the Emperor and his fleshsmiths to scribble in some extra instructions (as opposed to a woman's double Xs having no space, apparently. I have little knowledge about genetics and chromosomes).
I don't believe the OP was asking why there weren't any female Space Marines.
Basically because in a universe of daemons, basically magic, and incredibly advanced medical science that is basically magic, it seems odd that women are singled out of the Space Marine club.
With all that went into the making of the primarchs and space marines, seems odd to cut your recruiting pool in half.
Compared to the rest of the universe, it seems like it requires a reader to suspend their disbelief a bit.
Which I do, but it's still something odd.
People are just going to have to get over the fact that females are different than males.
Secondly once you're wearing power armor no one is going to be noticing that you have boobs.
Seeing as how boobie armor is extremely stupid and has never been done in history.
The blood angels chest armor thing is a different story altogether.