The term is fairly uncommon, mostly used in discussions of bisexuality to denote everyone other than bisexuals/pansexuals (with the exception of asexuals, who are not sexually attracted to any gender). It was likely adopted in place of unisexual, which is already used in biology and would produce confusion. It is often considered derogatory by the people to whom it is applied, and is not in common use as a self-label by either straight or gay people.
The proportion of people who fit into the category depends on how one uses the word. If the term is used to mean exclusively monosexual in behavior, then according to Alfred Kinsey's controversial studies, 67% of men and 87-90% of women are what may now be termed "monosexual" as determined by behaviour.  If the term is used to describe emotional response, the proportion is lower for men, 58%.
Freud thought that no one was born monosexual and that it had to be taught by parents or society though most people appear to believe that monosexuals are in fact the majority and identify as such.
Music critic and analyst Fred Maus  compares the criticism of Béla Bartók's works for their use of tonality and nontonal methods unique to each piece to the bias towards monosexuality and against bisexuality (see biphobia).
Among those gay men and lesbians who are familiar with this term, it is widely considered to be an ideologically loaded word intended to privilege bisexuality over other sexual orientations. Some in the bisexual community also avoid using the term for this reason.
In the early 1990s a Usenet flamewar raged for many months on the groups soc.bi and soc.motss over whether this term was heterophobic/homophobic, or whether it was simply the justified bisexual response to a frequently biphobic gay and lesbian culture.