I used to rent rooms in my house to friends who needed a temporary place and there was one summer when all the rooms were filled. It seemed like every night was echos of porn playing or one of our various escapades, since we were all single.
I just remember the somewhat shy roommate - me and my other friend were pretty open about when we'd score or if we were going to go off to have an epic wank session, but we could never get any info out of our other roommate. My other friend and I also walked around the house naked all the time, but our more shy roommate, while he didn't give a shit around nudity, always was fully clothed - maybe shirtless at the least. We always wondered what he was packing out of natural curiosity.
Well, one night, we got all the clues we needed. He wasn't too shy about bringing girls back to the place, and typically would make sure that they were being pretty quiet - I honestly didn't know if they were watching TV or whatever, it was that discreet. However, after a particularly rowdy clubbing night that we all ended up at, we all kind of spilled back into the shared home, along with a few other friends for regrouping on Ubers/taxis and probably to raid my bar for nightcaps. Once everyone left, only me and my two roommates and a recent college grad who we knew from her hangouts with our shy roommate stayed. We all wished each other good night and promptly fell asleep.
Flash forward an hour and I wake out of deep sleep to the sound of screaming. I bolted up and jumped into the shared hallway (we all had bedrooms in the upstairs of the house). As my adrenaline calmed down, I realized that the screaming was followed by heavy panting and moaning, coming out of our more shy roommate's room. While we always tried to create a chill environment and not get into anyone's business, I couldn't help but eavesdrop, especially since I could hear everything perfectly well from inside my room, they were so loud (probably still tipsy).
Well, we didn't have to wonder any more what our friend was packing - my cock went straight to attention as I listened to our friend's sex partner moan super verbally about his "hot body" and dick as he pounded the shit out of her, his bed creaking so loud. I stroked myself off while she screamed things like "it's so huge", "spread my pussy with that", "so thick" and wondered how he stayed so quiet when he'd fuck past girls.