The problem, from a purely in-universe perspective (whatever out-of-universe thoughts or bad pseudoscience might have justified it), is that the GoEM specifically designed Space Marines to be a one-gender race, or at the very least, incapable of breeding true. It's one of the few things about SM fluff that's remained 100% consistent - the Emperor wanted the Space Marines to serve and protect humanity, not replace them. Including both males and females in their ranks would threaten that, and put humanity in danger of being replaced by its superior version. So rather than just simply make them all sterile, he made them all sterile (outside of fanfiction :) ) and also restricted the design template to only work on males as a double failsafe (and because he was kind of a sexist bastard).
Eh, not really. There's many indications that they're made either sterile or mentally/hormonally asexual by the processes involved in making space marines.
I mean I don't think I've ever seen any fluff where a space marine has ever thought about or expressed interest in sex. So really, at least for me it's just the other side of humanity stepping up as protectors of the empire. True it's something that's remained consistent (making it kind of the exception all things considered), but apparently from what I've read GamesWorkshop has never done anything to really stop debunk it.
I mean they'd be just as zealous and duty-bound as their male marines in my mind. It's not like they'd suddenly become amorous chainmail... er power bikini wearing fan-service. I think some of the Sister o'battle artwork I've seen attests to GamesWorkshops ability to thoroughly avoid fan-service if they so desire. I mean, people are going to fap regardless of whether it actually exists within canon so I'd naturally need non-fappy reasons to even bother with this.
I mean, I'm hardly going to lose any sleep over, and I think people get a little too worked up over some silly things. If it continues as is, fine. I don't have the money to even play the table-top game proper so I'm only really interested in the fluff. So that means I'm going to postulate about stuff, what else is there really to do with fluff...
I mean... besides causing internet wildfires.
Nah, I wouldn't do that... would I?