So it’s a short and sweet story, not a wall of text. My older brother at the time was dating a fairly high maintenance blonde (he did not wind up marrying her). They were visiting, and because my bed was big enough, they were sleeping in my room during the visit (for future reference). So my brother and his gf were on one side of the family room, (divided by a TV cabinet) and myself on the other side, on the family computer, with head phones on. When I took them off to head up for bed, I could hear light moans from the other side. Even though I was 20 to 22 at the time, I clearly knew what they were doing, and so for several minutes I sat there, listening. When they were done, I gave a sigh of relief and went upstairs.
A few months or a year later, I brought this up to my mother. I forget whether my brother knew I heard them, or what exactly, but she was slightly disappointed with him, and according to her, she said my brother felt embarrassed by them having sex, trapping me on the other side. They did it there on the couch, so they wouldn’t get their fluids in my bed.