Whatever arbitrary set of categories you choose is still subject to the same subjective evaluation.  Your erotica is someone else's porn.  The same thing goes for defining "artistic quality" - Lord knows there've been enough threads debating that.  That old Supreme Court justice put it best, "I know it when I see it."  I find such terms of dubious value when their meaning is going to differ between groups and individuals.  If someone tells you an image is 'pornographic' it could be anything from a bare breasted Madonna to Debbie Does a Doberman.  Unless you know the person fairly well, you don't really know what they mean.  You can discuss what you like or dislike about something or how it moves you emotionally but IMO trying to categorize is pretty much a waste of time that only leads to endless circular debates and dead horse abuse.

I do wonder if Rodin had sculpted Ron Jeremy performing his signature act of self-satisfaction, would it be art ot porn?  

 

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken