The norm of monogamy is pervasive, having remained widespread, in most Western cultures at least, in spite of increasing tolerance toward more diverse relationship types. It is also puzzling. People willingly, and often with gusto, adhere to it, yet it is also, prima facie at least, highly restrictive. Being in a monogamous relationship means agreeing to give up certain sorts of valuable interactions and relationships with other people and to severely restrict one’s opportunities for sex and love. It is this restrictiveness which has led for calls among some philosophers for the justification of the norm, and some have argued that it cannot be justified. John McMurtry, for example, writing in 1972, compared monogamy to private property and argued that it serves the capitalist order (McMurtry 1972, 596–97). More recently, Harry Chalmers
(2019) has argued that monogamy is morally impermissible.
In this chapter we argue that rather than justify the norm, we ought to explain and understand its pervasiveness. Not everyone who follows the norm is doing so blindly, irrationally, or immorally. Indeed, many people are monogamous because being monogamous supports their needs and best enables them to flourish. We suggest that the human need for trust and attachment could underlie many people’s desires for monogamy and, therefore, explain monogamy’s pervasiveness. Focusing on the human need for trust and attachment security can also help us to understand why sexual infidelity can feel like such a betrayal. It can make sense to mutually agree to exclusivity in relationships, in spite of the apparent restrictions this brings. In a sense, not all restrictions are restrictive, because holding ourselves to limits on our behavior can in fact help us to invest in and experience the things we value most.