There seems to be a lot of prejudice and misconception about bisexuals both by straight society and by lesbians. From women saying they would never date a bisexual woman to people who believe bisexuality is only a phase or something trendy, discrimination against bisexuals is real.
Some lesbians might think that bisexuals "have their cake and eat it too." Meaning they take on the "B" label when it is convenient, but chose to pass as straight at other times. Of course this is not true of all bisexuals. Many bisexuals have to continually come out, especially when they are in an opposite gender relationship. Many bisexuals also work heartily for GLBT rights.
And of course lesbians often subscribe to the myths about bisexuals and don't understand what it really means to be bi.
