What rape culture in this case? If I saw my gf (not that I have one right now) sleeping with her legs open and a titty out, I'd find it hot and endearing (in a she's comfortable enough with me to sleep like that kinda way), and perhaps I'd go back to bed to cuddle with her till she wakes up on her own and see if said cuddling leads to morning sexytime. And I'd be extra horny from being awake in close contact with her (mostly or completely) naked body so it would be even better (or I might have to go jack off in the toilet if it doesn't happen). Has happened a couple of times, mostly when waking up to go to pee or something, not when I needed to go to work obviously.

I don't think any man who's not already way too OK with rape would just straight up go to town on a woman in her sleep without it being agreed upon before. Fantasizing about her and actually doing something about it while she's sleeping are very different things.

Of the women I've been with, one explicitly said she's okay with being woken up with sex (I didn't ask her about it or anything, she just saw me all horny in the morning so I somewhat jokingly said "oh I was tempted to take you while you were sleeping" or something and she got upset that I hadn't), and one said she's into it, but our relationship was pretty fresh and she didn't explicitly say she wants me to do it, so I didn't take it as consent as much as just her talking about what she liked or had fantasized about.

With the former I actually did it a couple of times, but soon realized that while her waking up suddenly to an orgasm was fun for both of us, the journey there is much more enjoyable if both parties are awake for it. With the latter, our relationship didn't last long enough (unrelated reasons) for me to even consider bringing it up for real.