On "Seinfeld," Elaine breaks up with baseball player Keith Hernandez because he smokes. In "The Boyfriend," Elaine and Keith develop a flirtatious relationship, but her strong dislike of smoking ends it when he takes out a cigarette.
The joke works because Keith initially appears to be an ideal match: he is famous, confident, athletic, and charming. Elaine's reaction shows that her personal deal-breakers can outweigh chemistry and celebrity. The breakup also fits the show's recurring habit of turning minor irritations into decisive relationship problems.
Keith Hernandez was a real former New York Mets first baseman, and his appearance gives the story an unusual blend of sitcom romance and baseball celebrity. The two-part episode also includes Jerry's friendship with Keith and the famous parody of the "magic bullet" discussion from "JFK." Elaine's later smoking-related behavior can seem contradictory, but it does not change the reason given for this breakup.