“The theater is about sex.” At least according to Richard Foreman, the father of the Ontological Hysterical Theater. “The Ontological Cowboy” documents Foreman’s invocation of the “manifest destiny” of the avant-garde theater, King Cowboy Rufus strolling down off San Juan Hill with a sigh, waving his handkerchief. Foreman plays himself, and the cast pantomimes his preoccupations.