Starring in your own o?

Several times I have read that when people start off in the business, they often star in their own movie. A guy basically pays some girl to have with him, films it and sells it. How is that not prostitution? I understand a company paying two people to have . That's different. But how do individuals, who may or may not have their own business, get around the legalities of paying someone to have with them, when there is no third party involved??? I don't get it.