"We don't exist. People who call themselves bisexual are "just confused" - if we came out properly, we'd be gay, or if we gave up trying to be cool, we'd admit we were straight."
There's this idea that everyone must choose between "straight" or "gay" - a kind of "taking sides". But why assume that everyone's attractions are gender-linked in exactly that way? Some people feel comfortable inside those categories, but let's not squash the feelings of the rest of us into boxes that don't fit.
"All bisexuals have both a boyfriend and a girlfriend, and can't be satisfied with fewer than two lovers."
In real life, some of us are happily monogamous; some are celibate or "looking"; some of us do indeed have more than one partner (though not necessarily "one of each"). Anyone in a monogamous relationship accepts that they may be attracted to other people at times without acting on the attraction; we're no different in that respect.
"Bisexuals have lots of sex all the time."
Identifying as bi is a different thing from wanting loads and loads of sex. Bisexuality is about the gender of the people you're attracted to. You can be celibate and still notice whom you're attracted to.
Some bi people, some straight people and some gay people have lots of sex (or would like to); some prefer to have little or none.
"It's only a phase."
Some people identify as bi for a while and later come out as lesbian, gay or straight. Some people identify as gay, lesbian or straight for a while, and later come out as bi. Some people identify the same way all their lives. Some people change their label more than once in their life, to fit a changing sense of themself. If it does change, that doesn't mean it "wasn't true" at the time.
"Bisexuals transmit AIDS."
You can't tell who has HIV by their label. A good start to protecting yourself is to learn the principles of how HIV gets from person to person and how to stop that happening.
(Note that what people often really mean by this is something like "a bi man might have sex with a dirty poof and then with a nice clean normal straight woman" or "a bi woman might have sex with a dirty piggy man and then with a nice clean pure lesbian". The implication is that as long as the favoured categories of people are kept safe, it doesn't matter about the rest of us.)
"Bisexuality is fashionable just now."
"Discovering" bisexuality as a current fashion makes a good excuse to write and sell a magazine story. And many people are curious, so they do like to hear about it, so the media like to run those stories every now and again. But it doesn't mean that prejudice against bi people has stopped. In fact, "you're just doing it to be fashionable" is a classic way of trivialising the subject and putting down bi people.
"Bisexuals are more openminded."
Not necessarily. Someone could treat either a male or a female lover in sexist ways. They could be racist, ageist, disablist and hold all sorts of other prejudices about people.
Being open-minded in the first place could help someone to come out as bi, though.