The bisexual menace consists chiefly in the fact that such people are incapable of making up their minds - when presented with options they will ask for both. This short education video describes the matter in somewhat more detail:
MFM threesomes are not as recent an invention as you might think judging by this photo taking from the archives of vintage erotica. We don't know when or where this was taking but if your granny or grandad is among those photographed we apologise and suggest you turn your back now: