Usually I just perceive it as a literary form of fanservice. Main character is a dude, must get sexy girls. And apparently nobody in the world of 40k is save, be them brainwashed assassins (who have absolutely nothing to gain from having sex with their handler) or nuns with guns (who are otherwise expected to maintain purity and isolation).
I blame the IP's power fantasy origins which, to this day, keep inspiring a large number of Black Library authors and result in what the fandom has colloquially dubbed "bolter porn". But I admit that it may just annoy me more these days as I'm already fed up with similar tropes in anime, and as such operate under the perception that romantic entanglements work to the detriment of a story more often than not, simply because the execution tends to focus on sexual fantasies rather than social drama. I'm sure there are examples that manage to do better (Song of Ice and Fire springs to mind), but unfortunately they seem to be comparatively rare, and indeed I cannot think of any from 40k.
... wow, apologies for the rant, I didn't want to steer the discussion that much into the OT.