Shots fired. Begun, the Lore Wars have.
https://youtube.com/watch?v=pPGPhKLbw-Q
And you know what? I've fucking done it.
You hear that GAMES WORKSHOP? You HEAR THIS Liberals, Cons and Faggots? I have solved the female space marine problem!
The 2nd Legion Astartes, Amazonia, were all Female. But there was a problem. Unlike their Brothers in all the other legions, they were actually fertile. They could have Children, and any child (Man or Woman) born of them held Dominant Astartes genetic traits. Without the need for gene seed implantation. This meant that given enough time the children of the 2nd Legion and their descendants would become the primary evolutionary apex of the human species.
This was horrific for the God Emperor, whose vision was solely on the restoration of natural, un augmented humanity to primacy of the galaxy. The 2nd Legion represented an existential threat to the grand goal of the Imperium of Man. So the Emperor ordered the Adeptus Custodes, backed up by the 6th Marines Space Wolves and their Primarch Leman Russ to exterminate the 2nd Legion, who much like the Thunder Warriors before them had been faithful servants of the Emperor before being betrayed.
Leman Russ refused to speak of what happened to the 2nd Legion primarch, Hypolita, because his betrayal of her is a shame he carried for the rest of his life.
There. Done. Female space marines. Perfectly Lore accurate without any pozzed Nu Warhammer bullshit. You are welcome everyone.
*edit* and now diving down this rabbit hole I've hit the gold mine.
Robute Guillemen has inherited a shattered Imperium. He needs force to drive back the end of mankind and his own bullshit codex astartese has stymied him. He needs a full Legion of Astartes. He has a feeling there may still be one but his memories are broken by his long stasis.
He just remembers that "the 2nd was not destroyed". For some reason when he mutters this in a meeting with the high lords on Holy Terra the Sisters of Battle in the room glanced at each other, ever so briefly...