I don't really see why people want female space marines myself, the Space Marines and Primarchs literally are made to be tools made by the Emperor for war and oppression and given how he treated the Thunder Warriors before them, they'd probably be eliminated as soon as he had no need of them. The Emperor considered all the superhumans he made to be nothing but lesser evolutions of himself, steps on the path towards the evolution he planned to be used then thrown away, the Space Marines in particular were created on a time crunch, and he had no intention of making them independent and self-sustaining. he just wanted an army to stomp everyone that opposed him that would be unable to stop relying on the very species they were created to conquer for and make dominant, so he went with the male thing because it was convenient for his purposes.

like, Space Marines aren't a badge of honor or anything, they're just some science project to make shortcuts for some immortal dictators vision of ruling the galaxy true, that since his glorified lighthouse coma, has been repurposed into defending a nightmarish feudal theocracy in space.

like. why would you want to portray that more positively by making it gender-equal? GW has explicitly said that the Imperium are not the good guys, not even a little bit. and like, aren't there tons of badass female characters in 40k who DON'T have complicated genetic enhancements and still kick ass? I'm not even talking about the female only factions. there are/can be female storm troopers, imperial guard, Inquisitors, Rogue Traders, Imperial Knights, the Officio Assassinorum, and of the actual female only factions the Sisters of Silence and the Adepta Sororitas. doesn't that kind of say that women don't need genetic enhancements from the manipulative space atheist genocide-dictator to be awesome?

and if you really need a genetically enhanced woman in 40k to kill a lot of things, Eversor Assassins exist. they are literally one person armies that are more powerful than Space Marines and meant to kill things in droves, all the while having no gender barrier.

but then again I guess if you can make something like the Eversor for both genders it calls into question how you can't for something like space marines, hm.....