A central life goal for many people, perhaps most, worldwide, is to achieve the monogamous ideal: a happy, successful, lifelong, monogamous relationship. Many also endorse monogamism: the belief that monogamy is the only ethically acceptable relationship arrangement. And many monogamists additionally endorse monogamous idealism: the view that the monogamous ideal ought to be a central life goal for the vast majority of people. Against these socially dominant norms, this thesis – which is primarily focused on the ethics of monogamy and (...) sexually open relationships – argues against monogamism and monogamous idealism and builds the case for the following proposals. We need to embrace, in a non-hierarchical manner, a plurality of sex and love lifestyles, including many different forms of sexually open relationships, various ways of living the single life, and monogamy. We should also have widespread awareness of the benefits, risks, and costs of all these lifestyles and recognise that no one size fits all. Consequently, individuals and partners can make informed choices with a range of options about how they wish to live. This thesis also defends these proposals by arguing against several conflicting perspectives. First, it demonstrates why we should reject various cases for (views similar to) anti-monogamism: the belief that monogamy is immoral. Second, it critically examines conflicting perspectives within the philosophical literature on how monogamy can be justified. Consistent with its main proposals, this thesis defends the idea that at least some couples can appeal to the difficulty of managing jealousy to justify their monogamy; and it argues that this justification for monogamy is preferable to the problematic justifications offered by other philosophers. Finally, this thesis argues against the following popular view, one that many monogamists, monogamous idealists, and those of a similar mindset endorse – the real-world view: although sexually open relationships are not inherently (or in principle) unethical, and while they are theoretically appealing for many, such relationships are (in the vast majority of cases) too ethically problematic in practice; thus, successful monogamy, despite its restrictions, is the relationship style (the vast majority of) people should pursue. (shrink)