In-laws in my case. Attending the funeral of my MIL at was a former neighbor and now girlfriend of my brother in law. Afterwards we were at a restaurant when the talk moved to the old neighborhood. That was when she, GF, revealed what she knew about the neighborhood. Seems old mom and dad were swinging with several couples unbeknownst to us, GF's parents included. We all sat there stunned as GF revealed what she knew which was a lot. She knew names of people who she saw come out of her parents bedroom in the middle of the night. Also knew who was hosting parties and how couples were paired up. It was quite an eye opening conversation.
I remember seeing my parents having sex twice and that one time where and when my baby sister was conceived. I saw my father eating mom's pussy - which lent itself to my confusion when he told me to never do that - and I saw mom sucking dad's dick - more confusion because she told me to not let a girl do that. Surprised? You betcha! Shocked? I can't say that I was because, as it would turn out and prove, they weren't doing anything that I wasn't doing with girls myself... and quite a few guys, too. I do remember one day when me and my three siblings were sitting and talking with mom about this or that and we let her know that, yeah, we used to hear her and our father (and her new boyfriend who'd marry her) getting it on and she had this embarrassed look on her face that I'll never forget!
Yeap, I remember going through puberty and I started hanging out naked in my room and experimenting with myself. That's when I got the talk from my dad. He and Mom were going to be having more sex and I should not be scared because it was a normal thing. He talked to me about my masturbation activities and I told him all I could think about was sex lately and I didn't think I was able to cum lol. He reassured me that it would happen eventually and I would get relief. From then on, I noticed every Friday night like clockwork they would get a little drunk and run up to their room (nextdoor to mine) and shut the door. I heard them talking, laughing and then moaning so of course I was in disbelief that they would do this while I was home. I guess they thought it better to normalize their sex and let me know it was happening. Anyways I am a curious person and found it way more interesting than I should. Imagine my surprise when I went up to the door , and I heard slapping and squelching noises, the headboard hitting the wall and my mother's obnoxiously loud squirting orgasms. Shortly after that I was able to cum... Funny how that dynamic works!