Sex is cheap. Coupled sexual activity has become more widely available than ever. Cheap sex has been made possible by two technologies that have little to do with each other—the Pill and high-quality pornography—and its distribution made more efficient by a third technological innovation, online dating. Together, they drive down the cost of real sex, and in turn slow the development of love, make fidelity more challenging, sexual malleability more common, and have even taken a toll on men's marriageability. Cheap Sex takes listeners on an extended tour inside the American mating market, and highlights key patterns that characterize young adults' experience today, including the timing of first sex in relationships, overlapping partners, frustrating returns on their relational investments, and a failure to link future goals like marriage with how they navigate their current relationships. Drawing upon several large nationally-representative surveys, in-person interviews with 100 men and women, and the assertions of scholars ranging from evolutionary psychologists to gender theorists, what emerges is a story about social change, technological breakthroughs, and unintended consequences.