No, it does not need to be a majority of sex work that is coercive to decide that prostitution should be outlawed. What number of exploited sex slaves in your country do you tolerate? Again, the difference is almost impossible to tell, which makes prosecution and hard numbers so difficult. Legalisation just means that forced prostitution can hide in plain sight. A lower ratio would just mean better chances of hiding.

I doubt that anyone could amass the evidence that you would demand, because for some reason you want to believe that there is virtually no forced prostitution where there is legal prostitution. Here in Germany, social workers and charities who work in this areas, even police and prosecutors, know that this is anything but true. It is hard to actually prove trafficking beyond a reasonable doubt.

But when you regularly see young Romani prostitutes from eastern Europe, addicted to drugs, having been obviously physically and mentally abused, lacking teeth, etc and still saying they are doing this out of there own free will despite not living better than if they were living on social security. Especially if their pimp is standing behind them. Well. It doesn't add up, does it?