Monosexual refers to someone who is attracted to a single gender. Monosexual refers both to gay people and straight people and is typically used by bisexuals, pansexuals, and other multisexuals to refer to anyone who is not mspec (with the exception of asexuals, who are attracted to zero genders). It is similar to the terms cisgender or allosexual in that way. It is commonly used when talking about monosexism; the prejudice against bisexuality and other mspec identities.
The romantic counterpart to monosexual is monoromantic.
Monosexism
Monosexism is the belief that monosexuality (either exclusive heterosexuality and/or homosexuality) is superior to or more legitimate than multisexuality.
Homophobes see bisexuality as morally wrong, as sinful, dangerous, or deviant. Some gay people dismiss bisexuals as lacking in self-acceptance, and claim that bisexuals are actually gay but are in denial. Others see see bisexuals as trying to maintaining straight privilege while simultaneously trying to get into LGBT spaces.
Another common attitudes toward bisexuality, especially among heterosexual people, is the fear that their bisexual partner will leave them for a member of the same sex. Similar fears exist among gay people as well, leading to the stereotype that bisexuals are more likely to cheat. There is a belief that multisexuals will eventually get married and "choose a side", dismissing the fact that multisexuals are still attracted to multiple genders after they are married.