The ONLY reason prostitution is illegal is because the government cannot control what goes on behind closed doors or the money exchanged for it. There are not any paper trails (invoices, deliveries, products or tangible goods exchanged). Therefore, the government cannot get taxes for it.
Look, we all know how terrible tobacco is for one's health and what the smoking consequences are to our health care industry. But, the government still allows cigarettes to legally be sold. Why on earth? GREED. That's right. The government is making billions of dollars a year in tobacco taxation and the health care institution along with insurance companies are raking in the profits of people being treated for cancer. But, tobacco is perfectly legal, nonetheless.
Cigarettes are easy to track and tax, prostitution is not.
IT IS ALL ABOUT THE MONEY. GREED.