Good question, we had a discussion similar to this in a graduate class I took. Of course, few people agree on what "art" is, but most people can agree on when art becomes pornography. I would probably say art becomes pornography when it is designed simply to arouse, or be deliberately graphic or shocking, rather than send a message of any kind. I don't think classical or Renaissance nudes (like David) were designed to incite lust or be deliberately offensive...but some may still be offended/aroused at ANY nude art. So defining pornography is easier but defining what "art" actually is, is much more difficult.
