Not as simple or as petty as that. Bisexual is an easy thing to claim when it's trendy. You get to berate and bully other people from within the alphabet minority. This is also the label that confused and closeted people use while they're figuring things out, so it's normal to have doubts when someone claims they're bi. Also bis have no real persecution. If homosexuality is outlawed they just lose half their dating pool / default to heterosexual.
Or ... and stay with me now ... when homosexuals say "same sex," we mean it. Gay men didn't want women before they started having mutilations to look like men, so why they hell would we want them after.