The Guardian UK has a nice piece titled, How We Ditched Sex And Fell For Thrillers, about the decline of sex-centered fiction and the rise of crime novels. An excerpt:"During the 1960s, a series of court decisions made it possible to publish just about anything. A burst of sex-centered novels followed. But about twenty years ago, many readers became sated with sexy novels. After all, sex is no longer the mystery it once was. And that's when the thriller entered the literary mainstream."
Then again, Freud would insist they're both the same thing, wouldn't he?