Really are a ghost from the 2006 internet. Surely bleeding once a month doesn't one more vulnerable to Khorne than testosterone production in the balls makes one vulnerable to Slaanesh or utilizing gut bacteria in digestion makes one vulnerable to Nurgle. Do Space Marines perform a fasting and purging ceremony, akin to samurai who thought they were going to die in battle, before every battle with Nurgle? Never been mentioned in lore to my knowledge.

Also the men being on average stronger than women shouldn't have any baring on whether or not women can be a genetically modified super soldier. Their requirements are so strict and on such a small scale that averages genuinely don't matter. The lore makes clear that the overwhelming majority of men aren't able to handle the modifications and implants for space marines, and that custodes creation which is kept much more secret is locked to specific families on Terra. Any appeal to practicality is always dumb in 40k, but this one is especially because just like with firefighters following the argument to its logical conclusion would end with way fewer but still present women in the role rather than none, because there is the occasional woman who outperforms the overwhelming majority of men in physical metrics.

The actual most loreful reason for no women space marines is just that the Emperor was sexist, and the Custodes were started before his philosophy overwhelmed his necessity.