Depending on who you ask, the response to the question “when did the sexual revolution begin?” will probably be one of two eras: either the 1960s, or, more rarely, around the 1920s or 1930s. Fr. Benedict Ashley takes the latter view in his autobiography (see p. 125 here). Illustrating this view, the Cambridge Apostles society of the 1910s and 1920s never made much of a secret of the promiscuous sexual relationships between some of its male members. And of course, it should be noted that the highly controversial Marriage and Morals by Bertrand Russell was published in 1929.
On the other hand, just look at the contrast from two excerpts from The Carolina Journal of Pharmacy, one from one era, the other from the other era.
The first is from 1934 (source):
The second is from 1972 (source). It is noteworthy that the article begins with the author saying “not only is this the era of sexual enlightenment, but it is also the age of contraception, legalized abortion, early sterilization, and great concern about population control … We are told we are living in a sexual revolution.” Then he adds:
This would seem to support the view that the sexual revolution really began in the 1960s. It is remarkable to see the change in attitude about the matter. In the first article, it is promiscuity that is the social menace, whereas in the second article, it is over-population. In the article from the 1930s, it is taken as a matter of fact that promiscuity is an evil, whereas by the 1970s, such a view seems antiquated (as it is still seen now). Overall, it is best to say that the sexual revolution occurred over a longer period of time than usually allowed, and it was more prominent in certain areas (e.g., England) earlier than in others (e.g., America, especially the more conservative parts).
It would be wrong to end a post about contraception and the sexual revolution without linking to a famous article from that period about that subject: Contraception and Chastity by Elizabeth Anscombe.