One can imagine that the authorities would have a great deal more resources to devote to sexual slavery and rape if they didn't have to worry about going after consenting adults agreeing to have sex in private.
Two comments on the article of this post. (1) The source is "Concerned Women for America." Feminist groups like this generally consider all prostitutes to be victims, taking the position that it's impossible to undertake prostitution freely a tendentious position at best. (2) The "Concerned Women" provide no statistics on the number of trafficked women. Why not? Two hypotheses: (1) there aren't any (which I suspect is true, if you demand any standard of validity). (2) the true number of trafficked women in the U.S. is negligible (which would totally undercut their case).
In any case, I read anything from a group with a name like "Concerned Women for America" as straight propaganda.