Sounds like an extremely mild case of sexomnia. I had it fullblown for an entire year. Would masturbate in my sleep quite often, and fuck my gf in my sleep a couple of times a week. Luckily, she enjoyed it and was fine with it. I'm a nice person when I'm asleep so when she didn't want to she'd just tell me to stop and sleeping me would go "oh, I'm so sorry, I didn't mean to.... Zzzzz..." without waking up. Of course I'd sometimes wake up but it was almost always a nice experience for me.

I haven't really determined why I'd do it, and why I stopped. I think it was stress related, because I was really stressed out in school that year. Had no sleep issues at all though, besides fucking my gf like a wild animal while unconscious.

People who suffer from sexomnia usually used to walk and talk in their sleep as kids, and as adults too. I never walked in my sleep, but I'd talk and thrash around in my bed, and still do. When kids walk/talk in their sleep it's usually because their brains are developing still so some weird shit can happen. When adults do it (including sexomnia) it's usually because the brain is unable to paralyze the body during sleep due to oxygen intake or the relevant area of the brain being slightly broken. I assume stress and other things could affect how well the body paralyzes itself. I'm not entirely sure though. So in order to increase oxygen intake, the brain will allow the body to move to get into a position where more oxygen is taken up. If that happens and you're dreaming about sex, while laying next to a naked, very potential sex partner, shit can go down.

Sexomnia can be very serious, however. Plenty of people who suffer from severe sexomnia feel terrible about it because you're essentially trying to rape the person next to you without knowing it.