For some reason I've been thinking of this all day off and on. What makes it porn?
For a while I was thinking that one thing all porn has in common is that it is commercially developed as a tool of arousal.
But, taking a step backwards ... if it informs, entertains, inspires, it's art or (not)porn.
If it arouses, it's porn.
And I don't think there can be a clear line between porn and (not)porn.
Porn is probably a word that was derogatorily used by some anal retentive sexually repressed religious nut, and the word I should have used is "eroticism" in my definition, and neither in my opinion is a negative word or a negative response.
I couldn't think of anything witty to say so I left this blank.