A society is characterized by the common attitudes and behavior of its members. Such behavior reflects purposive decision making by individuals, given the environment they live in. Thus, as technology changes, so might social norms. There were big changes in social norms during the 20th century, especially in sexual mores. In 1900 only six percent of unwed females engaged in premarital sex. Now, three quarters do. It is argued here that this was the result of technological improvement in contraceptives, which lowered the cost of premarital sex. The evolution from an abstinent to a promiscuous society is studied using an equilibrium matching model.