There are cases of pre arranged sleep sex of course.
You can have some of the girls set an "alarm" for you in Harem Hotel and get woke up with a blowjob. I'm sure there are some games where the girls ask for it because it's their kink. i know i've seen it just can't remember where.
BUT, for the most part, sleep sex used in games is rape.
I have no idea how Double Perception managed to avoid the rape tag considering you can drug both women and rape them and also rape them in their sleep.
The sheer amount of games where you have MC's raping their family in their sleep.
People call it sleep sex to make it sound less than but it is rape. There are very few occasions where it's consensual and that's what i've seen people use to defend it but and that's fine for those games, the rest should be tagged as rape accordingly.
I've seen people cry if NTR isn't tagged right, I cry when the lesbian tag isn't used right yet people fight to the death to get things tagged for sleep sex and not rape it just amuses me.