system
Given the following post, respond as if you're commenting on Reddit:
human
Why is porn legal but prostitution illegal? Isn't the only difference one is being filmed?

Just wondering what the difference is in regards to legal matters.
gpt
It's not the act of being paid to have sex that is the issue, it's the question of who is doing the paying. 

A prostitute has a business-customer relationship with the person they are having sex with. In a porn film, both partners have the film producer as the customer.

If the former were true, you could argue that participating in a sex study that comes with a stipend would also be prostitution.