Just because there is no "objective" standard doesn't mean there cannot be a common belief. I find the defense of some of the "art" now to be troubling in that fashion.

The vehemence to defend the most marginal simply means that people are wiling to suspend their consideration of the greater good in persuit of anarchy.

The care is in where the line is drawn...which is where most err on the side of caution...hence open displays of fesces in major museums and galleries.