I decided to look up why porn is legal and prostitution is illegal in the United States if you're interested I'll what I found on randazza.wordpress.com/2008/03/18/why...
which probably states this better than I can.
What I found makes sense, although I still think prostitution should be legal, it actually makes sense. The quick answer is that the companies pay the porn stars for acting, and then they have sex for free, since having sex is not illegal.
A supreme court case ruled this way after hearing a case against a video "Caught From Behind Part II" where the video had sexual scenes in it. The defending argument was as I stated in combination with the first amendment. I think the argument with the first amendment is just that they cannot stop them from distributing the porn because they have the constitutional right to sell whatever they want. They mention that it is illegal to film a person robbing someone else and sell it online and say "It's my first amendment right" because they can still be charged with robbing someone else and you can be charge as an accessory to the crime.
To get around prostitution accusations the court determined it was impossible to determine whether or not the people were paid to act and had sex for free or if they were just paid to have sex.
I like to think of it like this. I could pay Janet, the friendly neighborhood prostitute, to clean my house for 100 dollars. But god damn is Janet sexy and **** me if I just can't keep my dick in my pants and make a move. Guess what? She's receptive, must've been chemistry. Now I feel bad for what she has to do to get by, so I say "Keep the money and don't clean the apartment" the money is a gift and the sex was free and my apartment wasn't cleaned. You pay an escort service for company, and then if they just can't resist your charms and want to have sex with you, for free, that's ok. As long as you're paying them for company and not sex.
Hopefully today you've learned two things, why porn is legal and how to sleep with a prostitute legally. So if you ever want to sleep with a prostitute legally, offer her 100 dollars to clean your apartment, if she gets in the car she's not a cop and you can be honest, without fear of her being undercover.