This is why exactly you want to legalize it. Prostitutes tend to accumulate stds which they pass to their clients, which then pass those to other prostitutes. Upon legalization, they would be asked to take tests and those that failed would not be allowed to work so you basically start with a pool of healthy prostitutes.
On client side, they can also have the clients first present papers before availing service. Even if this is not strictly implemented, the diseased clients would be eventually weeded out. If any of the prostitutes caught something from a client, they can eventually narrow down to the possible culprit because of records and have those clients banned.
The biggest benefit will be the prevention of the human trafficking of minor girls. If the prostitutes are available legally, then the poor and rural girls will no longer be forced in to the flesh trade.
Oh yes, that's one more reason! Women fetch a high price for traffickers exactly because prostitution is illegal. Worse, the women are treated no better than slaves and gain nothing from their work. Legalization would end that.
Of course, this wouldn't stop pedophiles from going after girls but that's a different problem in itself. And just in case anyone would accuse me of wanting to legalize everything, no, pedophilia should be illegal. Children are too young to be asked to consent to sensitive issues like sex.
So guys, on everything you said, do you really think that legalization of drugs would help that people. Would it be better if every one had easy access to drugs and could do what ever they want with it? I know i would not want that. That would make some people life easier, and that people are bad for society. And i do not want to start on children, and how it would effect them...