Even with the current rise in awareness of sexual and intimate diversity, monogamous relationships remain the cultural norm. Most people aspire to it and the state encourages it, providing legal and financial benefits to married couples; however, statistics show adultery is commonplace, marriage rates are falling, and divorce figures are rising. This book traces the evolution and normalization of the monogamous ideal, questioning whether it is 'natural' or not, and surveying the spectrum of alternative relationship models that people are seeking out in a world of internet dating and scientific control over reproduction. It explores the emotional and psychological facets of ethical polyamorous relationships; questions whether these relationships benefit men disproportionately and whether they are compatible with raising children; and assesses the likelihood that diverse forms of multi-partner marriages and large friendship networks will become the norm in the future.