One of the most frequent justifications used for a sexual relationship with a patient is that it was consensual. Patients have free will, some argue, and both participants have a constitutional right to free association.
Many doctors are married to former patients.
It is paternalistic for the state or the profession to prohibit such relationships. And so on. The following document is an example of what one might use to elicit such informed consent. I found, however, that the document reflected the reality for many who had experienced such a sexual relationship, and I have been encouraged to disseminate it more widely. Read it, and ask yourself how many health professionals might be willing to solicit consent for a sexual relationship in this manner, and how many patients so informed might sign such a document.