The fear, hatred, aversion to someone who is bisexual in orientation.
Similar in nature to homophobia, biphobia is the unnatural fear of someone who is Bisexual or Pansexual in their orientation. The fear can be of the risk to health, or to being supplanted in their existing relationship by someone of a different gender.
The fear does not include nor exclude homophobia or other related fears, but is unique to persons of a bisexual orientation. In some manners, it is about the fear of promiscuity, of infidelity.
There are those who will be 'opposed' to bisexual persons for the perception that they are the carriers of the 'gay diseases' into the Heterosexual World.
There are those who believe that a person who is 'bisexual' cannot be monogamous, or faithful. That in essence they are more prone to be promiscuous.
Some who are Homosexual are opposed or discriminate against those claiming bisexuality due to the conception that they are merely trying to live in both worlds, or that they have not come to terms with their true orientation.
Heterosexuals will discriminate based on how there is only one acceptable sexual practice, that being heterosexuality. That the blending or mixing is not an accepted practice, or that a bisexual person is really a Homosexual, pretending to be aroused by a person of the opposite gender.
In short, bisexual persons are caught in the middle. There are also those who will claim that all people are really not constrained by any orientation, so that those who claim an acceptance of both kinds, are doing a disservice.
There are those who also believe that a person is either heterosexual or homosexual, and that no one can be bisexual, another form of Biphobia.
Noteworthy: Just as Homophobia is basic discrimination based on irrational fears, so is biphobia, only there are more variations, or stereotyping of bisexuals.