Funnily enough on one of my last encounters, which is quite a while back now, I had this topic. The lady was talking a lot about a friend of hers a lot, so I was jokingly pointing out that her profile said she wasn't too fond of 3P. She was quick to reassure that and that she wouldn't know what to do. She clearly had a very good idea what to do with a guy, but would have been completely lost if you add a girl to the mix.
On a MMF situation it's probably much easier for both guys to get their share of fun without interacting with each other, than it would be in a MFF situation for the girls. In other words, if the girl isn't bi interested, there is little motivation for her.