In May 2009 Billy stated the following regarding prostitution:
The main reason why human beings should avoid prostitution is the preservation of human dignity. To sell one's body or to buy a body is against human dignity. The sexual act between two people should be an act of love and/or affection.
Another important reason against prostitution is the dissemination of diseases (just as it is the case with promisquity).
There is no Creational law against prostitution; it's just a Creational recommendation.