AFAIK, "sex sells" mostly refers to promotional advertising. The origin goes back to as far as 1870s when a tobacco company used sexual entendres on package cover.
One of the most egregious examples was the National Airlines “Fly Me” campaign, featuring stewardesses saying, “I’m Cheryl... Fly me.” National passengers increased by 23 percent, twice that of any other airline.
Interestingly, there is evidence of this in newscasts as well. Regarding news broadcasting:“There are two sayings that are familiar in every news room across the country: 1) sex sells; 2) if it bleeds it leads.” The first is obvious, as is the second: if it's a tragedy, a terrible accident, etc., it's on the top of the newscast.
Before the actual slogan came out, though, sex sold. When one thinks of the 3rd c. work, the Kama Sutra, how many people think of its philosophies on family life and other advice on living a good life?