I have a friend, and our relationship is mutually open. It fits us better that way, and we both see other people occasionally. We are always honest about it. At one point, she wanted to have a series of quickies with new and different men, and she asked me if I would be close by just in case. It was a lifelong fantasy I guess.
I wanted no part of the voyeurism, much less participation. And she made it clear that her fantasy was to meet, take off their clothes, and go for it. Then, out the door. Comically, this appeals to guys in a way that catches them off guard. They use online profiles about how sensitive and caring they are...blah blah blah...and rarely believe that a legitimate quickie with a cute girl is all there is to it. I was always close by, and that fact was made clear to all. But it was a thrill for her.
After a handful of times she was over it, but I think she really enjoyed it as well.