If prostitution is illegal, how can it be legal to pay two people to have sex and film it in a porn movie?:
The difference between the two is more like Dolphins and Porpoises not Lions and Bats.

The reason one is legal and the other is not, is because the law says one is legal and the other is not. The actual difference between the two acts are hard to see. It is not legal to have sex with someone for money, but is totally legal to have sex with someone free. It is not legal to have sex with someone for money, but it is totally legal to have film two other people having for money if you sell that film commercially. A rational, logical human being would never be able to understand this, which is why it is something only lawyers and politicians could come up with.

The bottom line is they could make prostitution legal, they chose not to, they could make porn illegal they chose not to. It is one of those cases where the difference is only what the court says is the difference. Great question.