An attitude that privileges bisexuality above other sexual orientations.
This can be seen in attitudes such as "All people are bisexual by nature" and "Bisexuals are attracted to people's souls, not their bodies". It's not uncommon for bisexuals who have recently come out of the closet to take refuge in bisupremacist stereotypes as a reaction to negative experiences they've had.
Bisupremacy tends to be tolerated in support groups, but less often among activists or on usenet discussion groups like soc.bi.