Very naive post! Nudity != porn, in fact many porn sites have entire section devoted to acts performed by fully clothed women. Also museums do contain controversial art that most people would consider borderline porn, Marina Abramovics's recent exhibit at MoMA for example (http://www.nytimes.com/2010/04/16/arts/design/16public.html).
I think the correct approach towards an answer is to be found using something similar to the "reader-response theory" (http://en.wikipedia.org/wiki/Reader-response_criticism), i.e. porn is not an inherent property of a document but rather how we (the society) interpret it.
The question is nevertheless very interesting and, depending on your particular situation, could be more than an academic question: http://www.wired.com/threatlevel/tag/anime-news-network/ .
I think the correct approach towards an answer is to be found using something similar to the "reader-response theory" (http://en.wikipedia.org/wiki/Reader-response_criticism), i.e. porn is not an inherent property of a document but rather how we (the society) interpret it.