A monosexual is someone who is sexually attracted to one sex (or gender) only, monosexuality being this capacity for attraction or sexual orientation. A monosexual can be either heterosexual or homosexual.
The term is fairly uncommon, mostly used in discussions of bisexuality to denote everyone other than bisexuals (with the exception of asexuals, who aren't sexually attracted to either gender). It was likely adopted in place of unisexual which is already used for scientific discourse 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 heterosexuals or homosexuals.
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 Kinsey's controversial Kinsey studies, 67% of men and 87-90% of women are monosexual. If the term is used to describe emotional response the proportion would be lower for men, just 58%.
Sigmund Freud thought that children were born polymorphously perverse and had to be taught what he considered "normal" stages of heterosexual development by parents or society.
Among those gay men and lesbians who are familiar with this unusual 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 homophobic, or whether it was simply the justified bisexual response to a frequently biphobic gay and lesbian culture.
Monosexism describes a commonly held set of beliefs that exclusive heterosexuality or homosexuality are superior to a bisexual or pansexual orientation. For example, homosexuals and heterosexuals alike may dismiss bisexuality as promiscuity or as masked homosexuality, which might be deemed sinful or dangerous by a heterosexual moralist, and by a homosexual monosexist as a lack of self-acceptance.