That's interesting because my hubs and I did have the occasional threesome, but it was always with people we already KNEW, so there was a connection. With one friend it worked out well because she was a super cool person anyway and she was in tune with both of us. Very fun, but still, like three friends that decided to do something different that day.
One of the others-- not so much. She turned out to be not so interested in me sexually, but pretended to be-- or probably more accurately, was one of those "I kiss other girls to make the guys hot" ladies. The sex was uneven and I got really uncomfortable because of that. Now, had she just been more forthcoming, we could have just tag-teamed hubs and I would have been fine. But it was the "feeling snubbed" part that hurt, and in the middle of a sexual encounter, that doesn't go over so well.
We never really did swinging, though we did go to a swing club once for New Year's Eve. Hubs liked the open atmosphere, however he didn't really want strangers coming up to me, so we learned quickly that wasn't going to be something we could do. We didn't really meet anybody we were compatible with (i.e. would hang out anywhere else) either, so that's probably a big part.
Still... I have to at least LIKE the person, and to do that I have to know them on some level.