One of my best mates fucked my ex-wife when we were together. He was single and used to hang out at our place a fair bit, the three of us would just put some music on and chill out with a few drinks. One night we all got pretty ******* and were playing a type of truth or dare and my wife was guessing his cock size. He also knew we had been swingers, so offered to show her as he told us i was very thick but only about 6" long.
Anyway after a lot of nervous laughter/ giggling I said to her if she wants to suck him off, to go for it. She said she wanted to but in the bedroom, and only the two of them, while I stayed outside. It was that weird'ish dynamic that caused them to not have me watch.
Anyway, they went inside, couldn't hear much apart from a bit of laughter, then I heard the bed squeaking and her starting to moan, so I knew he was nailing her. We'd been friends for 10 years and she was always flirty when *******, so figured there was a bit of sexual tension/ desire there.
In the end, it was over in a few minutes, and he ended pulling out and blowing down her throat. And that was it, they came back outside with big grins, it was hardly ever mentioned again and they never asked to do it ever again.