This happened last year - I'm no longer dating the girl
Last summer, I was dating easily the hottest girl I've ever dated: brunette, tan skin, very voluptuous body. She always complained her ass was too big but I loved it.
So her parents have a lake-house that was an all-day's drive away. We'd been dating about a year and we went to visit one weekend, I think it might have been fourth of July. Anyway, her parents are the young-ish good-looking types. They had to have been the 'hot couple' back in the day...both of them still looking pretty good.
Anyway, a couple days into the trip I saw something I'll never forget. All four of us were in the hot tub/pool area and it was getting pretty late. I said good night to everyone and went to bed. I assumed everyone else would be going to bed within minutes. So I sleep for about an hour, and wake up needing to piss and my girlfriend's not in bed next to me. We were staying in the basement bedroom. It was a large, finished basement that overlooked the lake.
So I wake up, and walk to the bathroom. It was also in the basement. On my way to the bathroom, you have to pass these huge glass doors, and right outside the doors is the pool/hot-tub area. I noticed some movement, because there must have been a patio light on or something outside. I looked to see what the movement was and saw something I will never, ever forget: there's my girlfriend, still in her white bikini bottoms (her top was off), and her dad is lying flat on the ground. She's sitting on top of him, reverse, like 69 style, with her butt right on/in his face. He had his hands on her thighs, and she's just casually jerking him off with her right hand. I was so scared she'd see me that I just went to the bathroom, and back to bed. I woke up next to her the next morning.
From that point on, I didn't tell her what I saw, but that became my favorite sexual position after that: me lying flat on my back while she'd jerk me off in the 69 position, and I always thought about what I'd seen that night. We had a kind of nasty breakup a few months ago. I told her what I saw that night and haven't talked to her since. I still fantasize about what I saw that night.