Navigating the Ethical Dilemma of Romantic Relationships with Friends or Relatives of Clients

Having a romantic relationship with a friend or relative of a client. Where in the ethical code is this issue addressed and how might it be an ethical violation? Why do you believe the profession has chosen to view this issue as unethical some or all of the time? Can you imagine a scenario where you could argue that acting in such a manner could potentially be beneficial to a client?