I'm mostly just here to poke Ultrajoe. *pokepoke* Hi Ultrajoe!
I don't know as much about WH40K as I wish I did, but I do know it's already been retconned halfway to Dr. Who and back, and since all the shit about how Spess Muhreens are created is totally made up, all they'd need to do in order to overcome any real or perceived biological gender differences, if they wished to overcome them, would be to make more shit up. I have total faith in their ability to make up whateever shit they needed to make up, because if you can weaponize a massive blender full of psychics, there's really nothing you can't make work.
The idea of referencing anything in Real Life (tm) as a justification or delimiter of anything in WH40K makes makes me laugh until Psycker Smoothie shoots out my nose. No, the only legitimate delimiter on WH40K is the aesthetic of it. In this case, the aesthetic of the Space Marine is the lonely, fanatic, sacrificial isolation of those two masculine castes, the Priesthood and the Military. If you added women, you would add cross-currents and dualities, complications, and most importantly, remove the scary specter of Teh Gay that puts the proper finishing touch on those two Fine Manly Institutions.
No, we need to preserve the isolated and avoidant purity of that totally-not-gayness and that only works if you make it No Girls Allowed.
Rather than have Femarines, I'd much rather have something that takes the Feminine to the same lunatic extreme as the Spess Muhreens take the Masculine. Instinctive, primitive, swampily biological, not a scrap of hardware in or on them, they'd be pure bioweapons bonded by and communicating through a complex pheromonal signals. Every survival trick and trait found in the animal world would be present somewhere in the constantly mutating churn of their genome. As luridly colored as poison dart frogs, and for the same reason, their faces bear the aspect of Cheshire Cats, all wide, sharp-edged smiles and luminescent, predatory eyes, and their pendant dugs are glandular factories for everything from neurotoxins to molecular acids. They would enter battle not like militant ascetics but like Kali dancing, and they would scorn the idea of men among them. "No man," they would say proudly, "could ever survive what it takes to become US!"