Prostitution is sex for pay while pornographic films are films of people having sex. The reason films are not prostitution is because although the actors are paid, the person paying them is not getting sexual gratification from their performance and the players are not paying for the gratification they may receive. I add the following from one of my previous answers,
"In the state of Califpoornia, it is both legal to produce and distribute pornographic * films based on the Freeman cases, that permitted the hiring of an actor to have sex on the screen. As this was a California decision, it applies only to that state and remains the only state where such activity is legal. Other states may join and New Hampshire has been suggested. One would think other states such as New York and Nevada would be ripe as well. None the less, pornography is legal in many foreign countries.
As for internet porn, the Supreme Court, in Reno v. American Civil Liberties Union (1997) stated "indecent transmission" and "patently offensive display" provisions were ruled to limit the freedom of speech guarantee of the First Amendment leaving the internet porn industry an open door that some states have tried to close unsuccessfully. So at this time you can operate a site unless changes occur in the newly specialized field of internet law.
* by pornographic we are referring to the run of the mill fuck and suck films, not child porno which is illegal almost everywhere"
The defense of a prostitution charge by claiming it was a filming when the film maker had his cock in the actress's vagina has been tried and failed in court more than once.