Here is a very well written article that explores the difference between prostitution and pornography, and asks why one is legal and the other is not.

http://www.cnn.com/2005/LAW/08/12/colb.pornography/index.html

Consider again the significance of the sexual act: legal consequences can follow from it and it can, accordingly, be regulated by the law in a variety of ways. Though two people may very much want to have sex with each other in private, the law can intervene to say that they cannot, just because one of them seeks money and the other gratification, for example.

If, however, both members of the couple are in it for the money, and there is a man with a camera taping them, then the sex is insulated by the Constitution from legal regulation.