I'm not sure if I can add anything to this or not, but I'll be happy to give you my explanation of the "both of us or neither of us" sort of statements.
My honey is a quiet sort of guy and doesn't go out of his way to woo people. I tend to be very outgoing and talkative, and apparently people feel very comfortable with me. As a result I've been approached by several couples over the past 10 years that want to experience a fmf. While it's flattering for them to ask, and I'm glad that they feel that comfortable with me, I always turn them down when they say that they don't want a "couples" scenario. My take on it is that I'm not their playtoy, I am my honey's playtoy, and if my honey isn't going to be there getting off (whether it's with me, or the female half of the couple), ain't none of us getting off together.
We do play seperately, one on one. We don't keep score, I don't care if his little flavor of the month comes over once or one hundred times, and he doesn't care about how often mine comes over. But if it's another couple, then we're going to do it as a couple, or not at all. To me, it is selfish of the other couple to expect me to just leave my honey at home while I go off for a night of "wild monkey love", and it would be selfish of me to even consider it. No matter what the couple could do for me sexually, it would pale beside simply knowing that my honey was there having a great time, and being made to feel like a sex god for a few hours.
Maybe it's me being selfish about it, but if the couple is not willing to include my honey in our playtime, then I'm not interested in playing, I'll take my ball and go home.