So on the subject of female Space Marines. If GW put out female Space Marines today, I'd probably roll my eyes, but not stir up a fuss. That said, I dislike the idea for the following reasons:
1: From the idea of theme/motif (more the latter), female Space Marines really don't fit. We have the Emperor (male), create the Primarchs (all male), who regard him as their "father," and he their "sons," with Space Marine chapters venerating the Emperor not so much as a god, but as the perfect "man." In case you're missing the theme here, the Space Marines have a very...I'm going to say patriarchal, but I don't mean that in a negative way, I mean simply in the sense that the 'male motif' is clear to see. This arguably has theological parallels if we consider the Emperor akin to a god, and the primarchs as angels (FFS, Sanguinius has angel wings), or the traitor primarchs as fallen angels (you can draw plenty of similarities between Chaos and Hell). Just as the Sisters of Battle are matriarchal (female soldiers, female saints, etc.), the Space Marines are patriarchal.
2: From the worldbuilding standpoint, there's no hard reason why female Space Marines couldn't be created, but in-universe, it would be eyebrow raising as well. You've got over 1000 Space Marine chapters (all male), coming from Space Marine legions (all male), who've fought against traitor Space Marines (all male), and now, we've got Primaris Marines (all male). In-universe, some people (the High Lords of Terra I assume) have made the decision for over ten millennia to have their Space Marines be male. Which arguably makes sense (males tend to make better soldiers for a variety of reasons), but in-universe, having female Space Marines now...okay, why? How? If there was a time to introduce them, it was with Era Indominatus, but that's gone by, so yeah.
3: Finally, there's the question of motivation. If you want female Space Marines, why? From a model standpoint, conversions are a thing, most of your marines are wearing helmets, and if you want some helmetless female marines, stick Sisters of Battle heads on them. But that aside, I don't buy the idea that every organization in every female setting has to be open to both genders. It can be, and I'd roll my eyes at anyone also saying stuff like "but there can't be female knights, female knights didn't exist!", but keeping this in 40K itself, it's arguably telling that there's some people pushing for female Space Marines, but no-one pushing (as far as I can tell) for male Sisters of Battle (Brothers of Battle?). My personal view is that for tradition, both in-universe and out-universe, Space Marines should stay male, Sisters of Battle should stay female, and the rest of the Imperium is up for grabs. For instance, even if there was an Imperial Guard faction that was all-male in-universe, there's so many Guard branches that there's nothing to stop an all-female branch as well, because the Guard can be pretty much whatever you want them to be.
There's also another thing. I'd be at least sympathetic to people requesting female Space Marines if the demands came with interesting ideas for it (e.g. how would they relate to their primarch and the Emperor?), but I rarely see those ideas pitched. It's more "I want female Space Marines" with nothing to back that up that demand from a storytelling or worldbuilding standpoint. It's like...I dunno, to pivot to 40K's predecessor franchise Dune, it's like me demanding there be male Bene Jesserit without any actual rationale or ideas for how that works with the setting or with its themes/motifs.