One of the best films ever made is Frank Capra’s It Happened One Night, starring Clark Gable and Claudette Colbert. The 1934 comedy features an heiress on the run from her father, and the reporter who joins forces with her. The two fall in love and, alone in hotel rooms, to guard against temptation, they hang a blanket between their beds. They call it “the walls of Jericho.” When the couple finally ties the knot, the “wall” comes tumbling down. In the 1930s, a plotline that precluded premarital sex was a wise idea. Movie-makers who flouted the Motion Picture Production...