Some misguided observers of this debate actually believe there's nothing wrong with adults offering sex for money online. They ignore the fact that prostitution often involves other dangerous crimes, including robbery and assault. And sometimes these ads facilitate the illegal sex trafficking of women or girls who are being held against their will.
It is the case that forced prostitution is present in the United States. However, it is by no means clear that either a significant portion of prostitution is forced, nor that the level of forced prostitution would decrease when the difficulty of prostitution increases.
On the first point, we can take a recent empirical study on prostitution that requests no citation (citation will be given when the paper is published). What it indicates, however, is that there is a 63% increase in the number of tricks done on the 4th of July weekend. Forty-three percent of that apparently comes from full-time prostitutes. The other 20% comes from 4th-of-July-only prostitutes, part-time prostitutes that only come in because of the $11 average price increase. To Corrections, this sort of short-term supply elasticity, a vast reservoir that enters the market for one weekend only indicates voluntary prostitution.
If we assume that the 4th of July comes solely from a demand shock, that the fundamentals of supply remain the same, and approximating with linearity then we can identify the supply change (this is the source of the supply increase identification).
Below, we depict the pure data:
Here, we take the same data and add supply and demand lines, based on the idea that the 4th of July represents a demand shock:
On the second point, if we think that forced prostitutes are an inferior input into the production of prostitution, then enabling prostitution, causing an increase in the quantity supplied, will reduce their use in prostitution. The same concept might be applied to manual labor in manufacturing computers. It is likely that an increase from one computer demanded a year to several thousand would cause manual labor to all but disappear. So too with forced prostitution--cheapening the cost of supplying prostitution is likely to compete forced prostitution out of business.