From 2002 to 2009, I was active on swinger's forums, and also met couples from personal ad sites. There are some bi men on those sites. A small minority list themselves as bi. Then a larger percentage list themselves as straight, but they are bi (my guess is that between 25% to 33% are this way). In general, on the forums, and from what I've heard about on premise swing clubs, male bisexuality is done quietly, and not openly.
It has been my experience that bi males are much more accepted in the Austin poly group, than in the swinger's scene. That is a generalization. Usually, the swinger couples with bi males don't go to clubs as much, and tend to look for a small number of bi play partners to play with at home.