i have run across a ton of couples who only do MFF stuff. honestly, after doing this for a long time, i stopped playing with couples like that. i found they wanted a living sex toy and didnt really give a damn what i wanted and/or they were MFF only because there were issues, and even doing that caused more and i would wind up in the middle.
now i only go after couples that full swap with other couples as well.
i dont know a lot that only do MMF, but i do know plenty who are open to it in addition to the other stuff.
So, no, neither is unusual or anything. You'll have no issue finding men for MMF...finding reliable, good guys, might be harder. finding a single bi woman who is drama free and not flaky will be hard, but if you become friends with a couple, you might be able to do something where you guys switch off for 3somes sometimes.