I have been a serial adulterer and played around soon after marriage. Now in our 70s and still together after 50+ years, She found out and I stopped, but my business folded too.
I got over that and had a mistress who was my wife's best friend. In fact she propositioned me at a party and told me she wanted to fuck me while we groped in the dark dancing. I was stunned as she was the most glamorous woman I had ever met and I knew she had a bloke outside her seemingly happy marriage.
We were in an affair for years. She told me her hubby was like her bro those days.
He was amazingly good looking and pulled women wherever but never bothered and I just did my own dirty deeds basking in his attraction and pulling any of the girls who flocked round us.
Him and I would play tennis at a local club so the opportunities were constant.
She and my wife would go to our local pub now and then and I would babysit our two children. Her very handsome but sexless husband would do the same. One evening while they were out I had a wank and watched some blue movies as they were known then long before porn video and then had an idea.
I called the publican, who we all knew very well, and told him to give a bottle of champagne to them and saying it was from an admirer - I would pay him later.
They were well sozzled when they came back to our house. They sat side by side on the sofa and told me the story. I moved between them and just started caressing their knees then thighs and then crotches. They opened their legs and kept glancing at each other sort of checking, but of course my mistress was up for it, she was up for anything, so she took the lead and slid to the floor pulling me on top and we fucked while my wife watched.
The actual threesome didn't go any further as time constraints came into play and she had to leave.
In the morning my wife and I dsicussed the previous night without menace or recriminations and it was gradually forgotten.
My mistress told me they never did discuss that evening.