Even though writing about sex probably ranks on the joy scale somewhere between reading about it and having it, Elizabeth Benedict feels that many writers don't do justice to the act. So she has developed a novel idea: a guide book for fiction writers seeking to create better sex scenes. Benedict, a teacher in Princeton University's Creative Writing Program, doesn't concern herself with pornography but rather with a contention that sex scenes are pivotal in carrying the plot, story and character of some novels. Her point is emphasized through many interviews she conducted with authors on their experience with and views on writing about sex. Now, if she would only visit the film industry . . .