We consider prostitutes as social valves which helps to protect the society from worst things. (Don't ask now whether you like if your sister/wife/mother to be a prostitute) there are certain people those who want to have sex experience even with a prostitute. If they don't fulfill this, they may do some worst things like raping a innocent woman/baby etc (Still this happens in our country, what to do?) But at least one will prevent doing so by satisfying with a pros.
This is what we learn in Development Economics, But all these prostitutes must be a sister/mother/wife/daughter of someone.Now who want be to brother/father/husband of a pros?