I had a roommate in college who brought his girlfriend back to our room about 2am after I'd gone to bed. We had a loft above a living room and both of our beds where up there, separated by a small shelf. Nothing to really block a view or sound.
So they slip by my bed, of course I'm awake now, and then snuggle in. At that point, I'm like "okay they just want to sleep." Then within about five minutes, the girl starts moaning like she's being licked by a thousand tongues and then I start hearing the "slap, slap" sound everyone knows means two people are screwing. My roommate was pretty good at sex it seemed because it went on for about 30 minutes.
I was surprised, then pissed because it was late at night, during the week, I had early classes the next day and if he'd asked, I would have found another place to sleep that night. I got up for my 8am class, left a nasty note about it, and left.
Later that day, he claimed that she wasn't there, then backtracked and said, okay she was there but that nothing happened. Until then, I really liked and trusted the guy. He lied about something so obvious and then got indignant about it, telling me I was making it up. F-him. I think its just common courtesy to others to find a place to do your private bits in private UNLESS its clear the other roommate 1) wants to watch or 2) participate. Then its different.
Now, it was somewhat common (10 or so times a semester) for guys in our fraternity to do a "sport". This meant that guys hid upstairs in the sleeping loft, peering over the edge as the two got it on below on the couch. Obviously, all parties to this event were normally quite drunk. I never got to do it myself (either as the fucker or the watcher), but others did and told me that they were as hard as rocks. If there was more than one guy in the loft, I often wondered if one thing led to another and some guys got experiences they didn't count on.
One last experience to relate. Two friends of mine went to a rock concert (back when they had those) and picked up two high school seniors who wanted to party with college guys. Within an hour, all were upstairs in the loft, naked and doing the doggie. My friend who was more secure in himself said that he was more turned on by watching the other guy fucking his girl than he was by the high school girl he was poking in the ass. He said he could see the guy's cock glistening in the dim light as he went in and out. I assume he still fantasizes that that boy's cock.