In the same way, you have the _free will_ not to work, but the consequences of not working are ending up on the streets. As a special case of this, a prostitute has the free will not to work, on the threat of ending up on the streets. The only difference between a generic worker and a prostitute is that it is the human reproductive act itself that is being commodified.
I guess that the question of whether prostitution is exploitative or not, and whether generic work is exploitative or not, depends on what _alternative_ choices are available: Does society guarantee that you will not end up on the streets i.e. (1) is there a scarcity in the basic human needs of society, viz. housing, jobs, the recognition in law of that someone is a citizen etc. (2) does society provide these basic needs to all its members.
From this framework it's possibly to argue that _in principle_ prostitution is exploitative. It is no more exploitative than any other form of work, but perhaps has a tendency to attract the most vulnerable in society who do not have a safety net to fall back on. So I don't see anything especially exploitative about a middle class woman becoming a stripper to get through college. But not everyone is middle class.
What's undeniable _in practice_, is that legal prostitution leads to less criminal behaviour and exploitation. Hopefully we can work to change the conditions that make prostitution exploitative as opposed to an expression of an individuals creativity.