Hollywood was made to follow a moral code for films, which was created back in 1930 but not strictly enforced until 1934. Described as "industry moral guidelines," also known as the Hays Code, it was followed until 1968, and was the precursor to today's G, PG, R and X ratings. In 1915 the Supreme Court ruled that free speech did not extend to motion pictures; New York was the first state to have a censorship board (1921), due to the amount of lewd material showing up in performances. Read a full interesting history here, which includes images from films familiar to us today. The Code disallowed excessive kissing, firearms pointed at the camera, and a long list of plenty of other things.
From 1930 to 1934, the Code was not enforced due to staffing shortages, and in this time films like Frankenstein got through -- in violation of Code rules, but not yet censored. The Song of Songs, directed by Rouben Mamoulian and starring Marlene Dietrich, was another that escaped censorship, and portrays the destruction and debasement of a young woman's soul. Read a great write-up about it here, almost as good as watching the film.