Marriage and monogamy feature prominently on the public stage, but not all romantic relationships come in pairs. While people across the political spectrum debate the different-sex requirement of civil marriage, this article focuses on another limiting principle of monogamy's core institution: the twoness requirement. In particular, the article elaborates the practice and ethical principles of contemporary relationships of more than two people, called polyamory. Such relationships take many forms and aspire to several identifiable values, including radical honesty, consent, and the privileging of more sexual and loving experiences over other activities and emotions such as jealousy. The article asks why polyamorous relationships face such severe normative censure. After considering various objections, the article concludes that one key component of the censure is, paradoxically, the ability of most people to imagine themselves having sex with someone other than a primary partner - in other words, a paradox of prevalence. Despite the widespread desire for more than one sexual partner, however, and despite relatively widespread nonmonogamy, most people do not engage in polyamorous relationships. But neither do most people affirmatively choose monogamy: Laws and norms exert strong pressure on people to promise monogamy, and most people simply succumb to this pressure. The article argues that although monogamy is a sound choice for many, polyamory is a sound choice for others. Building on this premise, the article considers some ways that information-forcing principles of contract law might be used to help encourage people to make active, reflective choices about monogamy.