Why do porn movies not count as prostitution? In both cases, at least 1 party is essentially being paid for a sexual service.

… acting fees weren’t paid “for the purpose of sexual arousal or gratification, his own or the actors’.” (The sexual gratification of people who watched the movie was irrelevant.) The court also said that even if the actors had engaged in prostitution, applying the anti-pandering laws to skin flicks would impinge on the First Amendment.