The reports Mr Purple cited, one done 27 years ago and the other 16 years ago are flawed and do not have any real data behind them.
In fact the most recent TIP report used by the anti-prostitution and sex trafficking coalition has also been proven to be a misrepresentation of the actual facts.
There are way to many studies and articles published by credible researchers that prove the misrepresentations to list here so I will provide a link so everyone can make their own informed opinion regarding the percentage of prostitution involving forced commercial sex.
Flawed Sex Trafficking Statistics