Prostitution does not have to be exploitation.
There are most likely situations where prostitution is not exploitation, but I don't think they justify making it something normal. I'd rather live in a society in which we agree that some things are not for sale. Intimacy seems to me like a good candidate for something that is not for sale.
We want people to stop objectifying women, let's not have womens' bodies routinely sold and bought.
Perhaps it is the best way for her to get ahead and escape her current situation. People do it anyway and now they have almost no recourse when they are being manipulated. If it were legal, women would actually have more power in the situation.
Consider also according to studies, a majority of voluntary prostitutes have been victims of abuse as children.