The definition of pornography need not be rigid, and I don't think it's possible to delineate it by content alone.
Pornography fundamentally is exaggerated sexual activity (or suggestion) construed in a way that will arouse the audience.
But why does that require penetration? The solo shoots in Hustler fully qualify as pornographic, even if no penetration is taking place.
In fact why does it even require nudity? You could film two people 'dry-humping' fully clothed, moaning, and talking dirty to each other, and most of us would agree it's porn.
I think when beauty and composition come into the picture, then you start to have an element of art -- and there's no reason why something otherwise pornographic couldn't cross over into that realm. Maybe the artistry of it would make it 'not quite porn' and the physical banality of it would make it 'not quite art', but why does there have to be a strict boundary between the two?