This is not properly formulated. In fact, the imagery in the Sistine Chapel is predominantly nude and looking upon them is not a mortal sin…neither for the newly elected pope, for the cardinals electing him, or for the countless people who visit the chapel on a daily basis.
I am at a loss as to how gazing upon Michelangelo’s David in the Uffizi could be declared a mortal sin even though he is unabashedly nude and anatomically correct. I have no doubt, sadly, there are people who sin in their hearts when looking at it…just as they can with any other thing they gaze upon. That, however, is a problem internal to such people.
The criteria is: is the response to be evoked an inappropriately sensual one or not. Doctors and nurses, after all, confront nudity every day and do so without sin.
As the original poster says, a person can be fully clothed and yet the photograph, painting, or the person in real life can elicit a more sensual response than any Michelangelo that is completely nude.