After further inspection I'd like to point out that John doesn't actually seem all that jealous. Just because the narrator says he is doesn't mean his actions point that way. He leaves Alisoun alone with Nicholas and he lets her listen to Absolon's love song.
Perhaps John is simple "sely" or naive, rather than jealous. He says he loves her more than his life, so maybe John is just blinded to her betrayal because he loves his wife so much. That might be a better moral to the story. He still cares about the earthly world (his wife) more than the spiritual world and thus he is punished for it, but Nicholas doesn't get off easily either (although I think I'd rather burn my butt than break a bone).