I saw a porn movie the other day where this guy was doing his girlfriend while she was sleeping. Is that really possible?
Is it possible to have sex with someone while they're asleep? Sure it is.
Is it likely they won't wake up at all? Not so much.
Is it likely that person would wake up and think it was awesome someone was engaging them in sex without their permission in advance? Probably not.
More to the point, is it okay to have sex with someone who is asleep without having asked and obtained their permission first at a time when they were NOT asleep? No way.
(Does most porn care about presenting the idea that sex without consent isn't ever okay, and that when people have sex with sleeping women without their consent, they are not inclined to wake up and do anything but look for something to bash that person in the head with? Not so much, but all of us should be smart enough to know not to be gleaning ANY sort of sexual ethics from pornography, ever.)
Sleeping people cannot give consent to sex, because we're not conscious to make those decisions, and having sex with someone who cannot give consent isn't lawful or ethical, nor is it even sex: it's rape .
You could ask me to do all of the things I LEAST wanted to do in the whole world when I was asleep, and I'd grunt yes just to get you to shut yer yap and let me sleep already. It's often hard to even know if and when you're in danger when you're asleep because...well, you're asleep. I'd expect someone I trusted enough to share a bed with to know that they shouldn't put value in ANYTHING I say when I'm asleep, and to ask me about important things -- sex included -- only when I was awake. So, if you're interested in discussing the option of initiating sex with a partner when they're asleep, that's certainly something you could talk about in advance of doing it, but you still won't really have full consent at the time. Too, if the idea of having sex while you're asleep is exciting to YOU, then it makes more sense for you to talk to a partner about them doing it to YOU than you doing it to THEM.
Overall though, it's just not something I'd advise, even with a discussion first.
Giving a general permission to a partner to do something that we can't soundly decide on when it's being done isn't so kosher. Too, it can be awfully creepy, and quite an invasion, to wake up and find someone -- even someone you like and trust -- having sex with you when you've been asleep. Really, it tends to be way more creepy than sexy for the sleeping person (and you'd hope, also creepy for the waking person, since two people can't actually have sex TOGETHER when one of them isn't conscious). We need a feeling of safety to sleep soundly, and it'd be awfully tough to have that if we felt like someone could have sex with us when we weren't really aware of it, or not able to give clear consent to it, at any time.
Your best bet, here in real-life land where consent matters? Enjoy snuggling in sleep, and save the sex for before you go to bed or when you and your partner are waking up. Just woken-up morning sex is often mighty nice, and we're still bleary enough for it to feel good and floaty, but not SO out of it that we can't make a sound choice about whether or not we want to have sex.
Here are a few extra links for you to check out: