There are lots of bisexual women in San Francisco who have boyfriends and there are venues to enable the fabled threesome. Pleasure Zone is the only one I know of that is having regular, public events; but there are other private parties for couples and single women (no single men are allowed). Other regular coed events to find available bi women are the fetish and swinger scenes.

Disclaimer: YES, I know that being bisexual does not = wanting to be involved in threesomes. I, however, quite enjoy them.