Pornography is in the eye of the beholder; but so is art. In my view nudity in itself is not pornographic per se.
It is all about quality, taste and in some extent intent.
Is Michelagelo's David porn? Are Boticelli's paintings porn? If not why not?
Pornography and art are not mutually exclusive. You may argue that Henry Miller's books are pornographic, but they are also considered works of literature.
What you find on porn sites is almost all not art, but then most of the stuff some photographers label as fine art isn't either.