In college, I had roommates. One year, I lived in a small old bungalow near campus with a couple. They had sex a lot, and didn't seem to notice, or care if I was in the house. I'd hear them in the morning, at night, and in the shower. If they came home really late after a party, they were especially loud. The headboard of their bed would slap the wall and bang on the radiator. She was very vocal in bed.
At first, I thought they were just caught up in themselves, simply oblivious to the fact that I might be in my room, studying. Then, as it went on, I started thinking maybe they were being rude, maybe even a little passive aggressive. But that didn't make a lot of sense, because otherwise, they were nice enough. We all got along fine.
Then I realized why it was so loud in the house when they had sex. They never shut the door from their bedroom that led into the bathroom. All the sounds from their room carried into the bathroom, and was amplified, like an acoustic guitar. Maybe it never occurred to them to shut that door, or maybe they even wanted the sound to travel, or for me to one day have to use the bathroom, and come in, and look up into the mirror, and be able to see them. And that's exactly what happened.
If I stood at just the right angle, I could see through their open doorway into their room. He was lying back on the bed, and she was gently, but eagerly slurping on his cock. Even semi-erect, he was huge. No wonder she screamed so loudly as he pounded her. How could I not help but watch?