Monosexual refers to someone who is attracted exclusively to a single gender. Monosexual refers both to exclusively gay people, straight people, and anyone else who is attracted to a single gender. It is typically used by bisexuals/multisexuals to refer to anyone who is not bi+ or m-spec (with the exception of people who are both aromantic and asexual, as they 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/multisexuality.
The monosexual flag was created by Tumblr user beyond-mogai-pride-flags on November 14th of 2018. It has no confirmed meaning.
Monosexism is the belief that monosexuality (either exclusive heterosexuality and/or homosexuality) is superior to or more legitimate than multisexuality. It is, at its root, a form of biphobia.
Monosexists 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 bisexuals as trying to maintain straight privilege while simultaneously trying to get into LGBT+ spaces.
Another common attitude toward bisexuality, especially among heterosexual people, is the fear that their bisexual partner will leave them for a member of the same gender. 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.