Culturally, it is acceptable for men to have many straight sexual encounters and it is ok for women to be a little bisexual. (Think of the sterotypical porn movie.) So a MFF threesome is considered mostly acceptable by society and so I think that is why bisexual women are in high demand.
However, I think we should try to think past that and figure out what is right for us in our moment. Maybe it is a V or W? Maybe a MMF triad? Maybe swinging?