I love couples and swinging with them. I've been meaning to get out again with a particular couple in mind , but have been short on time with the holidays.
I have been to many venues in the lifestyle. I have never seen any MM activity. However if you look on SLS.com or SDC.com there some couples that list themselves as bi-male. It is my belief that they tend to play among themselves.
I do know that at the house party setting that I attend on a regular basis, any suggestion of this would likely result in asking that person or couple to leave.
As I think is has already been stated, most but not all of the ladies are bi.