When I was in college, many moons ago, my roommate and I set up the room with bunkbeds. I was a virgin, but my roommate had enough sex for both of us. He was captain of the swimming team who grew up in Hawaii, so you can imagine his build. His specialty was the butterfly, so his chest was massive. Picture a 6 foot blond with broad shoulders and a 26" waist.
Anyway. This one night I was asleep and was woken up by the bed shaking. I had the top bunk, and my roommate had tucked a blanket under my mattress to serve as curtains for his bottom bunk giving him total privacy. While I couldn't see what was going on, I could certainly feel it. At one point he must have put his feet against the springs in my bed and pushed up. He lifted my bed with me in it off of the frame and down I came on top of them! He and his date du soir had a surprise climax, and I think I had my first threesome?