Napping aside, when (or where) did "leaving your desk for lunch" become taboo? Is this an American thing?
In most places I've worked it would be unusual to stay at your desk for lunch, and in many places it would be discouraged. People really need to use their lunch time, however long or short, to actually take a break, not just from work but also from sitting in front of the computer, and maybe get a few minutes of fresh air and natural sunlight.