So umm if you invited a single female and she acted like a whore and tried to take your man away, would you not swing at all then?
Or how about this, you invited a single bi-fem and she tried to seduce you, would you not swing anymore?
IMO couples prefer MFF trysts because the husband/bf doesn't want to see another dick, worries about his SO getting attached, and wants to fulfill his fantasy first. After all two ladies playing with each other is gentler than two men playing with a lady according to the common stereotype.
To me, couples are more understanding and forthcoming to let me play than a single female. I prefer it this way because the pressure is off on me and I can have the husband/bf keep her attention from waning while I enjoy her at my own pace.