Mann Act
In the 19th century, in many American cities, people could only sell sex for money in some areas of the city. These were called red light districts. During that time, cities became more urbanized; also more women started to work. This meant that there would be more cases, where courtship could take place without supervision. In this environment, a concern of "white slavery" started: the fear was that women would be  kidnapped and forced into prostitution. In 1847, Charles Sumner had described the Barbary slave trade. The ideas originally were from this description.