This is indeed a difficult subject. While I agree to some extent with your statement, I'll point out where I see the errors and slippery slope:
The government does rightly legislate morality (what believers regard as sin). Examples are murder, theft, rape, extortion, robbery, burglary, etc.
I agree that calling it "sin" is riling to unbelievers, yet, at it's core, we have these morals because we recognize these acts do not benfit the victim, they are not victimless crimes.
I do not believe, (and I would gladly debate the subject) that anyone would have sex for money completely willingly. If someone chooses to fornicate, pleasure, not making money is the natural driver. In a perverse materialistic society, the lines become blurred, and making easy money by doing something enjoyable robs, perhaps very slowly, and so without relaization, the prostitute of their character and "soul", their dignity, their right to be loved for who they are rather than what they will do for money.
As I stated, it's a difficult subject. I do understand your point you are trying to make about it not being advocacy, in it's superficial intent. However, I believe it boils down to indifference and lack of compassion to not support the protection of (yes, even the willing) from destroying a key part of their humanity that they can not regain once they have gone down that road.
Prostitution is not just another job, it does something that is fundamentally dehumanizing in order to accommodate instincts that in a truly ‘better world,’ would be channeled into more fruitful and dignified relationships.
Wow BA, that was deep. You've changed my mind about legalizing prostitution. Wow.