The "rushed and shitty" explanation is literally canon. The Imperium was a oh-shit-oh-fuck moment by the Emperor to try and unite humanity to stave off total extinction by smashing the major threats to humanity's survival while also collecting enough resources to breach the webway so that Old Night couldn't happen again when the warp gets too fucky to traverse.

Of course the Emperor is also an idiot who failed to realize that Chaos would never allow such an outcome, so all of this blew up in his face and now everything is terrible forever.

Astartes are all male because Big E was both desperate enough and utilitarian enough to think that it'd be easier to make geneseed work for the sex that already starts off with more physicality and aggression by default for his rabid disposable shock troops than to try and make sure it all jives with any and all possible recruits (and is totally fine with ludicrous attrition from the implantation process even with those parameters, because they were never going to stick around long term in his vision anyway). Nobody's fucked with that since because the original work is considered sacred beyond compare and the only people both smart enough and free spirited enough to fuck with it are either desperately trying to pump out even more Astartes to keep the Imperium crawling along longer and don't have time to futz with the gender settings or are busy creating a new race of superhumans that does include all genders.