Exploitation is still exploitation, even if the victims agree to be exploited. This is something we in the U.S. have a very difficult time grasping because we have been taught that exploitation and commerce are one and the same, and that freedom means being free to exploit those who are stupid enough, weak enough, or unlucky enough to let us. But this is not commerce, and it never was. It's exploitation. It's taking advantage of one person to the advantage of another.
Anyone who sells their body to someone else, to be used for whatever selfish purpose, is allowing themselves to be exploited. That they are economically compensated for it may mitigate their feeling of humiliation, but it does not change the facts. We should not legalize such exploitation for the same reasons that we would not allow the exploitation of children, or the exploitation of the mentally or physically challenged. And the reason is that they are unable to properly defend themselves due to specific conditions and circumstances.
People who fall into prostitution are already victims, whether they know it or not, and they need to be protected from the abuse and exploitation of others just as anyone else who is not able to protect themselves needs to be protected by laws. We don't allow adults to have sex with children just because the child agrees to it, nor do we allow adults to sexually exploit the mentally or physically challenged, even though they may agree to it, as well. And the reason we don't allow it is that we understand that the agreement is the result of exploitation, and is therefor unfairly obtained.
Same goes with prostitution.