It's always amazed me that we seem to try harder to protect children from seeing a nipple than we do to prevent them from seeing depictions of people being killed.
Our beliefs about other people's beliefs affect what issues we'll complain about in public. With the result that there is more care taken in our ratings to prevent children seeing sex than violence.
Edit: Found it: Pluralistic Ignorance. http://en.wikipedia.org/wiki/Pluralistic_ignorance. Uncited, but interesting statement: "...Pluralistic Ignorance can be caused by the structure of the underlying social network, not cognitive dissonance."
"Groupthink is a psychological phenomenon that occurs within a group of people, in which the desire for harmony or conformity in the group results in an incorrect or deviant decision-making outcome"
Pornographic or erotic imagery is different. It taps into a natural impulse that almost every person has and most likely doesn't promote particularly virtuous deployments of that inclination. Sex, and by extension sexual imagery, has a very powerful pull on most people and the use of sexual impulses must be tightly controlled if reasonable social cohesion is to be maintained.
Will and Ariel Durant, prominent historians from the mid-1900s: "No man, however brilliant or well-informed, can dismiss the wisdom of the laboratory of history. A youth boiling with hormones will wonder why he should not give full freedom to his sexual desires; if he is unchecked by custom, morals, or laws, he may ruin his life before he understand that sex is a river of fire that must be banked and cooled by a hundred restraints if it is not to consume in chaos both the individual and the group."
I find that description extremely apt. Sexual imagery provokes a more fervent reaction because it has a much more dangerous effect on a much wider segment of the populace than violent imagery.