I've heard a roommate fuck his gf, but it wasn't a turn on, b/c he did it intentionally. Him and I were college roommates and didn't get along, so he brought up his gf and fucked her while I was in my room. Just one of the battles in the war we waged.
The year previous to that, I had a roommate (I got along with this one) bring up his guy friends who were utter and complete a-holes! So, him and I shared a bathroom which was b/n our rooms, well, I had to use the bathroom and upon walking in, I could hear one of his friends litterally fucking the daylights out of a local high school girl. The slapping was loud and judging by the volume of the slapping and the thumping on the floor, I'm guessing she probably had some bruising the next morning. Oh, and to top it all off, the guy left his used condom on the bathroom sink, which I discovered the next morning. Needless to say, that experience wasn't a turn on either.