Another contributing factor to the "I don't know any bisexuals" thing is that some people refuse to acknowledge someone as bisexual even when that person specifically self-identifies as such. I've had a number of friends (some of whom I've known for many years) tell me they don't know any bisexuals, and when I'm like "um, hi, how long have we known each other?" I get "oh, well you don't count because you're married/you don't act bisexual."