Hi.
I've participated in both types of threesomes. The first was FMF with my friend and her husband, and it just happened. We were comfortable enough with each other as friends that he started playing with her and the next thing I knew, I was involved. It was very cool.
The MFM was with Mr. Precocious and my friend's husband, and it too just happened. Mr. Precocious started playing with me and the next thing I knew, the friend's husband was involved. It too was very cool.
When I think about fantasies, the thing that makes them fantasies is that you never know when they might happen. Spontenaiety is important for me/us, and making new friends and seeing where things go, if they go, if part of the fun. But then, where we live there are no swing clubs. There are house parties and we typically don't attend them (we feel that play is expected). There are Meet and Greets at bars/lounges where sometimes couples leave paired up. We haven't left in that way yet and really don't want to.
For us, the "perfect" experience is something that just happens. Planning, scheduling, etc., just make us feel awkward, but maybe that's because we are still very new to this whole thing.
I would say that no pressure should be placed on the third party, but when something is planned, there is pressure. When you are looking for that perfect experience, there are performance expectations. When things just happen, there is no pressure or expectations.
Just my limited experience opinion.
Mrs. Precocious