The term compulsory monogamy refers to the pervasive assumption and belief that monogamy is the "natural" or best way for humans to be in romantic or sexual relationships with each other. The notion that monogamy is the ideal and that everyone wants to be in a committed, monogamous relationship with one other person leads us to presume that any two people who are dating or in a committed relationship must be monogamous. This is not always the case, and whether or not monogamy is more commonly practiced than other forms of relationships, compulsory monogamy is a false assumption.
It is interesting to note that the presumption of compulsory monogamy persists despite an enormous amount of evidence to the contrary. We know that both humans and non-human animals thrive in a variety of types of relationships. We also know that many people are in committed relationships that allow for partners in the relationship to engage in and/or maintain emotional and/or sexual relationships with people not in the committed relationship. We also know that many people in relationships they call monogamous are in fact having sexual and/or other kinds of meaningful interactions with people outside their primary relationship.
Read more about compulsory monogamy:
Emens, Elizabeth F. Monogamy's Law: Compulsory Monogamy and Polyamorous Existence. University of Chicago, Public Law Working Paper No. 58.
Mint, Pepper. Compulsory Monogamy and Sexual Minorities By Pepper Mint. Accessed May 22, 2011.