Obviously, there are significant differences between prostitution and pornography. However, from a policy viewpoint, it does become difficult to draw out those distinctions in a way that makes sense of our current situation--one where two consenting adults are breaking the law if they contract for sex, but aren't breaking the law if they contract for sex on camera, with the intent of publicly distributing the resulting images for monetary gain.
Regardless of the genuine distinction between porn and prostitution, it's very difficult to see how the policies that lie behind the prohibition of prostitution should not, if they are valid, also support the prohibition of pornography...in fact, even more so, since porn is so visible and is not, ostensibly at least, carried out in private space or for the sake of private interests (the sexual gratification of the sexually involved parties, for instance).
From a legal standpoint, the difference in treatment between porn and prostitution has little to do with a difference in policy considerations or with the general differences between the two categories (though, as I've said, those differences do seem genuine and important).
I'd explain the differing treatment as more of a historical accident, fueled by this country's precedent-based judicial system and economic factors. At some point early in the game--very likely due largely to historically local motivations--some judges decided to put sexual imagery on the protected side of the 1st Amendment line. That decision put the power of precedent into play, and as a result, a massive economic institution--the porn industry--was allowed to come into existence.
Today, the industry undoubtedly takes sexual imagery far, far beyond what the judges who initially set the precedent could have imagined. Nonetheless, the power of that precedent, heavily bolstered by the now economic impossibility of dispensing with the porn industry, keeps it and its output securely under the umbrella of protected speech 99% of the time.
There are real and important differences between porn and prostitution. But those differences don't provide the best explanation for the differences in how the law treats the two. Rather, that explanation lies in the historically local motivations of a group of judges quite some time ago and the influence that current economic realities exercise upon current judicial outcomes.
imo, at least
