Every individual has the ethical right to provide any service they wish to provide in return for cash as long as the service does not involve physically harming anybody. Therefore, every individual has the ethical right to provide sexual services in return for cash.
Although I believe this right should be protected, I do not think buying/selling sex is a necessary business. Sex is free. Sex does not have to be paid for. Anybody dumb enough to pay for sex is throwing their money away.
Although I support a person's right to sell sexual services, I do not support any person for selling sexual services. Prostitutes are pointless because they are selling a service that does not need to be sold. Something that is free should not be paid for. Furthermore, paying for something that is free should not be supported or promoted.
I do not believe it is ethical to prohibit somebody from selling sexual services, but I do think there are ethical methods of disipating it through awareness, support, and promotion of free sex.