Specifically, monosexism promotes exclusive gynephilia and androphilia, regardless of the sex of the person.
Some heterosexual people define bi- or pansexuality as morally wrong, as sinful or as dangerous, while some go further, describing all orientations other than heterosexual as such. Some homosexual men dismiss self-identified bi- or pansexual men as lacking in self-acceptance, and claim that such men are actually gay but are in denial. Lesbians who make dismissive claims about the behavior of self-identified bi- or pansexual women tend to state that such women are actually heterosexual, and are engaging sexually with other women for attention from men, or out of confusion.
Monosexism refers to prejudice against bisexuality specifically. While it describes a view of the superiority of heterosexuality and homosexuality, it is not the same as heterosexism or homosexism which are views that include each's superiority to the other.