Dating patients is considered unethical by the profession of medicine (at least in Canada) because of the power differential in the doctor-patient relationship. The theory is that since patients trust physicians with their health and well being, physicians could manipulate patients and coerce them into a romantic relationship, which is obviously unethical (and pretty gross too!). In Canada, physicians are self regulated by the Royal College of Physicians and Surgeons (RCPS). The RCPS decides if a physician can practice within a given province, and they deal with any ethical complaints that have been raised against a physician. Thus, if a patient complains about a doctor coercing them into a relationship, they would raise that complaint with the RCPS. The RCPS would then investigate the accusation and decide if the physician is guilty. If the physician is found guilty, the RCPS also dictates the terms of their punishment, which can range from rehabilitation or suspension, to revoking the physician's license to practice.
In most cases, the rules against dating patients makes complete sense. However, as with any ethical, political, or moral stance, there are always gray areas. The other day we were taught that the RCPS has a zero tolerance policy with regards to the doctor patient relationship. This implies that if you are the only physician in a small town, you will probably be unable to date ANYone in that town, as they're most likely all your patients! That means it's a TERRIBLE idea to go to an isolated rural community and be the only physician there, unless you enjoy celibacy.
Another interesting fact is that the RCPS believes in "once a patient, always a patient". This means if you are on call and you see a patient who comes into emergency, you can NEVER have a relationship with them. If you see them again in a social situation, 4 years later, and hit it off...you can't act on it because one night, 4 long years ago, they were your patient.
Now, in practice, I'm not sure if the RCPS is as strict as we were told they are...but if so...wow...