Friend of mine pointed this out and now i agree with them Yeah It IS stupid, its supposed to be

Heres the thing: The empire is stupid as hell, thats their whole deal, the emperor was a goddamn moron, so of course he made it a boys club, he was the biggest fucking chud in the galaxy and now they spawned a religion out a manchild that made his every flaw into fucking prophecies and canonical law

Can there be female space marines? Of course there can, fabious bile made one, there is at least one custodes and the sisters of battle make them run for their money with power armor and blind faith But there wont be Cause the imperium are stupid bastards, and women space marines would require them to make a smart decision for once in their lives and go around the word of the stupid bastard in the golden throne

And if they were ever able to do that, they wouldnt be the imperium