Why is prostitution illegal?

Why is prostitution illegal? This law doesn't make sense considering strange men are allowed to approach women and ask for free sex. People who don't know each other hook up every night. Anyone can catch a disease if they don't use protection. And as far as violence, domestic abuse takes place between boyfriend and girlfriend, and wife and husband. An adult should be able to decided what they do with their body. And I'm not meaning go out and find a pimp or solicit in public, but to work for yourself as a freelancer in a private location.