Uptight Bible-thumpers that are only afraid of the fact that seeing a nude woman makes them feel uncomfortable so the only logical reaction is, "Hulk Smash!"
{edit} I should add, "and the bureaucrats that are afraid of litigation/boycotts/bad publicity from the Bible-thumpers"
{edit2} I could include Quran-thumpers too, but they don't have huge representation in the US. Maybe moreso in Europe though?
Winning an argument with your boss or HR person over whether it is art or not is pointless when you're being fired. Nudity in the workplace is generally frowned upon. Whether that is right or wrong is another point entirely.
Porn is a different matter.