Prostitution is a voluntary sale of a labor service. If both adults are consenting on the terms of the services then it should not be a problem.
How can you say its voluntary? It is a problem because most of these girls don't have a choice. Take Bangladesh for example, the girls are illiterate and either get married to an nasty buddha when they hit puberty or are forced into prostitution. They're both equally disgusting (Astag). Or take the man/woman who doesn't have a choice and follows in their parents' footsteps. It's still not voluntary in my eyes.