>>96208998 (OP) So we're going to need to break out some feminist theory here.
So there's a concept you've probably heard of. The male gaze. The tl;dr on it - it's making shit that's meant to appeal to men (consciously or unconsciously.) If you want to know more, there's been a lot of ink spilled on this concept.
The sisters of battle are made within the male gaze. They're hot battle nuns that are sometimes scantily clad for pure, religious, definitely not sexual reasons. Sometimes there's maybe some implied sexual tension between the women but any lesbianism present is there largely there to tease the reader's dick. They're uncorruptably pure women until they're not and you, the player, can enjoy the fantasy of despoiling something pure.
Now we're going to step out of academic feminism and talk about just general stuff. "The ick" is a term used by a lot of women to describe when something in unappealing in a way that's hard to vocalize. It's a pretty broad concept, and applications of the ick are entirely personal.
Now we intersect these two concept. SoB have the ick for women. They see the sisters of battle, and even if they don't know everything that's going on with them, they can pick up the vibe. They can pick up that while this is the army of women, it's an army FOR men. To be clear, there are sometimes going to be women who like the SoB. Personal experience is that trans women love them more than cis women but that's a different kettle of fish.
Now. You mention why Orks, Tyrannids or Slaanesh. This can only be anecdotal. But let's get something out of the way. Women who play 40k aren't normal. They're fucking weirdos.
So then we break it down. We start with 'nids, the army I see most women playing. They're fucking monsters. I do not say this lightly, women fucking love monsters. Not always in a sexual way, but sometimes that's part of it. But having some kind of creature that knows only hate? Women eat that shit up.