Legal theorists have long debated whether law should enforce social morality. This paper explores a different problem: should law (try to) improve social morality? I argue that it should. First, against conceptual and empirical doubts, I argue that it is possible for law to improve morality. Second, against certain moral objections, I argue that it is often proper for law to try to improve it. Third, I offer an example: law should try to improve our social morality of sex, by trying to re-shape what we regard as valid consent to sexual activity. Along the way, the ideas of H. L. A. Hart and Patrick Devlin are examined, as are the empirical and policy claims of Paul Robinson and his collaborators. A revised version of the paper is to appear in Criminal Law and Philosophy.