Prostitution is arguably a victimless crime that is only made worse by being illegal.
Due to being a criminal offense in almost every state, prostitutes cannot get the help and protection they need from the goverment. According to a study in San Francisco, 82% of prostitutes are assaulted and 68% had been raped (Farley, "Prostitution, Violence, and Posttraumatic Stress Disorder").
Sex workers also have no regulation when it comes to health standards. Nevada, where prostitution is legal, requires sex workers to get monthly syphilis and HIV tests. They also are required to participate in weekly gonorrhea and chlamydia tests. Not only that, but condoms are required for all forms of sex.
Prostitution is not a problem, the legality is. So why not make "the world's oldest profession" legal?