The medical school interview may break the rules of etiquette for the purpose of finding your true character and identity, as well as to see how you would react to situations with peers or patients. As a physician, you must be able to endure such discussions with ease.
If you read "Princeton's Guide to Medical Schools," there is an interview section, which discusses possible questions asked at interviews, and many would qualify as offensive. They are nonetheless asked, and there is nothing wrong with this, in my opinion.
The question is less aimed at finding out this information, and more aimed at how you would react to such a question. In all cases, when a question is asked that one is not comfortable with, a declination to answer is always an acceptable response. On the other hand, if the interviewee "threw a fit" and "was embarassed," this would show how they would react as a physician, if such private matters were to be brought up from a patient.