Non-marital sex in the United States — pre-marital, extra-marital, and post-marital — went through a radical change over the course of the twentieth century. It both increased markedly in frequency and availability and changed dramatically in character. The pace of change was most rapid in a period of perhaps ten years beginning in the early 1960s. What accounts for this revolution in such a core human activity? Why and how did we shift from a world in which sex outside of marriage, while not unavailable, was not nearly so abundant, was generally unreliable, tawdry, and, for women, usually shameful, to one in which it was readily and widely available, seen as part of the natural course of unmarried life, and where the failure to engage in sex with some regularity was generally viewed as harmful and shameful?I am aware of no fundamental change in the drive for sex or its ontological and psychological meaning that explains this revolution. I will spend most of this paper providing an economic explanation of this transformation, and particularly its pace in the 1960s. I will focus primarily on the plummeting costs to women of engaging in sex. That waning of cost came in two different, but related, dimensions. First, there was a dramatic amelioration and even reversal in the negative material consequences of sex. Second, and perhaps more importantly, the reputational price that women had to pay for engaging in non-marital sex plummeted in a manner analogous to a neighborhood “tipping” model. Beyond exploring the causes I am concerned with the consequences of this change in practice and attitude. I will discuss the immense negative externalities both pecuniary and real that have come in the train of our more libertine practices and attitudes. That I will suggest that the earlier more prudish and chaste sexual culture had a number of broad societal benefits that have now been eroded will hardly be surprising. Grounding such an observation in the language and concepts of economics will, I hope, prove informative and thought provoking.