In college, I had two guy roomates who dated self-identified lesbians. I asked them once, why they identified as lesbian since they were in a semi-monogamous relationship with a guy. (One couple just cheated on each other a lot, so it was semi-monogamous.) The basic answer I got was that they wanted to stay in the lesbian culture and bisexuals were not quite welcome in that group. Being bisexual seemed to be a sell out.
That saddened me that a group/culture founded on sexual freedom represses some forms of sexuality. In the end, we are individuals that rarely fit neatly inside labels.