All of my wife's extramarital sex has been with my friends, and I encouraged it. I've always told her I'd be comfortable with her having sex with anyone she wants to bed, old boyfriends, guys she meets when she's out and about, whoever she fancies really. But she has stuck to having sex with my two closest friends--one of them for about ten years--the other for 30 years and counting. On a couple of instances she even took both of them to bed at the same time.
It's easier with friends, really, You already know them, like them and trust them. And, in our case both men were married much of the time, and their wives never thought twice about it when they said they were coming over to see me, so no awkward excuses that might fall apart under scrutiny were needed.
Sometimes I'd watch. Sometimes I'd participate. Sometimes I'd deliberately absent myself, as one of my friends said my wife was much less inhibited when I was not there. After all, I wanted her to enjoy herself, and I found I really got off on imagining just how wild she was with either or both of my friends when she was unencumbered by worrying if I thought she was having better sex with them than with me. We had pretty good sex, but I loved the twinge of jealousy whenever I thought about either or both of them outperforming me. Even more erotic was the thought that she was giving more of herself to them than me.