Asked my best friend, who is a general practitioner / family doctor about this.
He said that while he notices, he is expected to be thoroughly professional, and not show any reaction whatsoever. He then stated that he has seen on his patients every type of underwear imaginable as well as a few commandos.
His only comments were to a teenage male who was wearing a cockring. He asked the patient's permission to ask him about it. The patient agreed, whereupon the doctor explored the reasons why he was wearing it, et cetera. He concluded the conversation by warning the patient about the dangers of wearing a cockring for extended periods of time, and the damage that could ensue. And for the first time, because he had opened a good rapport with the patient, complimented his choice of Unico briefs (they were almost bikini, and with a suspensory pouch). The patient smiled, left, and it was never discussed again in subsequent meetings with the patient.