So, what is acceptable without any sort of code or standards?
I could, say collect orphaned children and mentally handicapped folks, and prostitute them for profit. Without any caretakers, I could become the caretaker and give them a place to live... and train them to the point that they wouldn't want to leave.
In fact, if they are mentally handicapped enough, I could train them to say that they have the capacity to contract, and there's no authority to judge these sorts of things, so therefore, it would have to be assumed that my contracts with them were binding.
Children would be particularly easy to convince to enter into contracts... I mean, I could create an awesome place for young teenagers to go that might be slightly angsty and have problems with their parents... I could get them to claim they are old enough to make rational decisions and that they understand long term consequences... and all they need to do get out from under the mean old boot of their parents, and into my swanky mansion full of parties, drugs, arcade games, and sugary foods would be to just occasionally take their clothes off for the cameras.... No big deal... after all... that sort of thing is acceptable now that prostitution is legal.
(Did I mention I can play a wicked devil's advocate?) There has GOT to be a better answer than these individual little points....