In the Wall Street Journal , Eugene Volokh tackles onerous state laws that prevent, for example, an optician or dental hygienist from marrying a patient. The piece rightly assails such laws, but I'm not even sure I'd agree with Volokh's one concession:
Of course medical relationships offer room for various kinds of abuses. In some situations, it may be proper to interfere with people's right to marry, and their sexual and romantic autonomy, in order to prevent those abuses. We can talk about relationships between psychotherapists and clients (or ex-clients), or relationships between doctors and current patients, or other circumstances in which the risk of subtle coercion or unprofessional behavior is especially high (which is to say materially higher than the risk of subtle coercion and other harms in any sexual relationship).
While I'd agree with the general sentiment here, I'd argue that even the most obvious example of abuse—a psychiatrist sleeping with a patient—ought to be a professional ethics matter, to be adjudicated by peers on medical boards. I can't see any reason why a relationship between two consenting adults should be any business of courts or legislators.
Somewhat related, ESPN: The Magazine has yet another tale of age-of-consent laws gone wrong.