Ok, so hi guys. This is actually a real story (thats why im posting it here) and was the reason why I got into this fetish. At first I was into cuckold but not this specific "branch" if we can call it so...
All this thing happened like a year and half ago. It all started with my mom getting invited to a  meeting of former high school partners. It was taking place on a nice place with pool, grills, a nice garden, etc.
I went with her, not to the meeting but to a party of a friend who lived close by. I accompanied her to the meeting to waste some time because it started early, like at 12 PM and my friends party was at 2 but I didnt want to travel in the bus.
In the meeting was this guy, the owner of the place, catching a lot of female attention because he was handsome, athletic and muscular and lets face it the last two features are way uncommon amidst 40+ years olders (the guy was 44). He was divorced and with a good job and lived like he was in his 20s, partying a lot, training hard etc. At the end there were few people at the meeting, about 25 people or so.
I went to my friends party and stood there for a while. I ended leaving up early since I was returning home with my mom because both the meeting and the party were at the outskirts and it was a long travel on public transportation.
I returned to the meeting place to wait for my mom. When I arrived it turned that all of the guest had left and there was only my moms and the guys I mentioned before cars.
I peeked and saw my mom KISSING with the owner of the place. But i mean, kissing hard. They were both sitting and hugging in the garden outside the pool, my mom in a one piece swimsuit and the guy only in shorts showing his chiseled body. As I said they were kising each other and they started breathing heavily, my mom saying he was so hot and the other answering the same, he started searching for a way to remove my moms swimsuit when her phone rang.
It turned out that a friend of my mom (who was also a guest of the meeting) had forgotten a blouse and she called to see if my mom was still in the place. After mom finished talking she said to the guy it was too late, that she better left, etc. I ran away and my mother called me like five minutes after to ask if i was returning with her. She picked me up some blocks away from the meeting place and she let me drive because "was a little drunk". I think she was saying the truth because she smelled a bit like alcohol but her face was blushing and seemed very umcomfortable. I was too.
Its the first time I mention this to anyone. I dont know what I would have done had they started to fuck. I felt a little upset while watching, but I felt mostly umcomfortable. After a few days I started to think about the "what if" scenario in an arousing way, and so I ended here.
I dont know if they have met alone, but my mom has him on her friend list on fb.