It is often assumed that romantic relationships will inevitably be monogamous. But what does monogamy mean? This chapter points out that the rules around monogamy have become increasingly confusing and contradictory in recent years, and partners often have very different ideas about what constitutes ‘cheating’ or ‘infidelity’. Also globally monogamy is actually less common than non-monogamy, and many people are really secretly non-monogamous in western societies. Perhaps monogamy could better be understood as being on two spectrums: from emotional closeness with one person only to many, and from sexual/physical intimacy with one person to many. This chapter explores what we can all learn about relationships from people in open and polyamorous relationships, as well as aromantic people and relationship anarchists. It also considers the diversity of relationship forms, including non-human relationships, and what it would be like if we treated lovers more like friends, and vice versa.