The documentary Adult Entertainment: Disrobing an American Idol did a reasonably good job of poking holes in every pro-porn argument except one based on fact that free expression is a public good. But I think such an argument fails too, since porn simply is not the act of someone expressing himself.
If I hear someone who watches a lot of movies say “wow, that was a great movie” I assume the thing he is talking about had a features like an engaging plot, characters with a good arc, and that it hung together as an interesting whole. To hear someone who watches a lot of porn say “wow, that was a great porno” doesn’t suggest any of these features. Plot, character, and even cohering as a whole are necessarily irrelevant to porn – one could spool out the film and sell it by the pound without changing the product one is selling, and it’s certainly strange to say that one is expressing himself or communicating a message when one can dispense the message like this. There are media other than pornography that have this sell-by-the-pound quality, for example, those relaxation recordings of ocean waves or creek-waters running over rocks- but the comparison only sharpens the point, since we don’t make those recordings as acts of self-expression or artistry (what kind of artistic skill does it take to mic the ocean?) but only to induce a state in another. Recording the ocean is not analogous to the arts but to pharmacology: it is a relaxant. The same goes for porn. Forget being art, it’s not even best compared to art but to stimulants: amphetamines, drag-racing, bungee-jumping, etc. At this point, one could argue that it is a harmless stimulant – just don’t call it expression, except in the vacuous sense that anything one does can be an act of self-expression.