Milfy City, Dreams of Desire, Long Live the Princess, Echoes of Lust, Midnight Paradise.
well there's your problem. you're mainly playing incest games. so the question really is: why is sleep sex so common in incest games. and the answer is: because they live in the same apartment, and sexual interest is repressed because of social norms until the characters buckle under the pressure. the characters always start with "this is wrong, I shouldn't think of my mother/sister/son like this" but they can't stop themselves. first they sneak & peek, then the other party gets into giving handjobs with poor excuses etc and claiming she's not into it, then she drops the denial and they become lovers.
is it always well done? obviously not. and often it doesn't make sense because the dev either doesn't include the sleep progression in the 'daytime progression' or the LI flips from 0 to full slut instantly. but that's a writing problem, not a logic problem.
My main problem with sleep sex is not that what the MC is doing is wrong. its that he is stupid and in most cases a delusional hypocrite that thinks he is actually a good person.
this contrast comes from his denial of his sexual attraction to a family member. doesn't mean he's a good guy, but the whole point of his character arc usually is the transformation from a goody two-shoes into a pervert. again, something that's often not written out well, because most of us are shit at writing.
frankly I can never understand the frequent complaint of not liking the MC because he 'thinks' he's a good guy but then does bad things like molests his mom in her sleep. that's called a complex character, and is universally thought to be a good thing in a story. han solo starts his arc with a straight up murder because he doesn't want to pay his bills. james bond is a serial rapist who habitually raped the female villains, and not in a 'soft rape' way but holding them down kicking and screaming until they stop resisting. clint eastwood in high plains drifter rapes a woman kicking and screaming before he saves the town, because he's a 'hero'. you get the point.
but also the fact that the MC is doing these bad things to his mom/sister proves he's NOT the good guy the complainer usually claims he him to be. so again the complaint makes no sense.
but I do understand how often these arcs are written poorly, the self-doubts and gradual corruption isn't portrayed clearly. but again, that's a writing problem, not a genre/character problem. the idea is 100% solid, but good ideas get fucked with poor technique.
the real question though is, how would you like it done differently, without removing the fetish? khumak suggested waking the girl up so she can show willingness, but I think that's a sort of vanilla view to a fetish you don't share. it's like saying "why can't you make NTR where the MC isn't humiliated", which can't be done because it wouldn't work for the people who do have the fetish. it's how a vanilla person sees NTR and how he'd 'solve' the 'problem' of him not being turned on by the fetish. by removing the fetish and replacing it with vanilla sex dressed into the outer signifiers of the fetish.
the only way I see the waking up (without stopping sexy times) working in this context is if you make it a straight rape scene, but I don't think most of sleep sex fetish people would like that. it goes to a different category, the rape fetish where you overpower the victim. not the same thing, and probably a huge dealbreaker for everybody else. a small overlap of people into both sleep fetish and rape fantasies would like it, but that's a tiny group unlike voyeurists.
