http://www.ted.com/talks/zimchallenge.html
This TED talk is a favourite of mine on this topic. Without needing to refer to a moral framework I'm fairly confident that porn is harmful to people for the fact that it inhibits us from experiencing other important joys in our life.
It is arguably bad in the same way junk food is. Not because it is immoral, but because it's counter productive to longterm health and happiness.