My ex was a sexsomniac. I woke up one morning to him humping at my rear end, and I tried desperately to push him off and get him to stop, but he didn't wake up until I screamed in agony. He was very well-endowed, and just woken up, on your side, trying frantically to wake someone up is not the best position for anal.
That was my first and so far last experience with anal and penises, though another time he had been fingering me, and accidentally jammed two fingers up there. There were lots of screams and some bleeding.