XXX

At The Atlantic, Tony Comstock is running a history of movies. His current write-up is concerned with the years 1968-75 and how the X-rating became associated with porn:

Where moviemaking is concerned, the word, "adult" has fallen on rather hard times, having become code for works that are sexually garish, possibly exploitative, and amateurish in execution. But it wasn't always that way.
...

The producers of Starlet! leverage the market's understanding of what the X rating means by describing their own offering as XXX, borrowed from the old practice of using X, XX, or XXX to denote the strength of beer. (Leave it to the Aussies to give us XXXX beer!)

The following year sees the release of Midnight Cowboy with an X-rating. The film is nominated for six Academy Awards and wins three, including Best Picture. In fact, it has been suggested that out of concern over the impact of the depictions of drug use and sexual content in the film on children, United Artists studio boss Arthur Krim self-applied the X-rating to Midnight Cowboy over and above the R-rating the MPAA planned to give it; and this is supported by the fact that the MPAA granted an R-rating to the film, without cuts, only one year later.