Saying your 'bi' is just a way to ease your fan bases shock/discomfort at the sudden change in your percieved sexuality. Its all about the molah.
But a lot of people genuinely are bisexual. anyone who says it doesn't exist is a huge hypocrite. You can't complain about being discriminated against because you are gay and then scoff at bisexuality. I am a bisexual girl and a few lesbians have been pretty rude about it. Most of them don't care, but a few of them get uptight about it.
The LGBT community has some sad in fighting sometimes, lesbians and gay men criticising each other etc. Doesn't really help things move forward.