It's not a simple matter.
I think that we've gotten too accustomed to looking at human trafficking from a purely legal perspective. When we see what prostitution does to the lives of women, we see that it doesn't really matter whether we refer to it as trafficking or not. The Palermo convention sought to define trafficking as situations in which people are displaced, their documents are withheld, or conditions are imposed on them so that they actually become slaves or entrapped.
Regardless of what led these women or men to enter the sex trade, we have enough information to know that prostitution itself has a major impact on the lives of women, who are the ones who are primarily concerned, obviously. Trafficking is another element that adds to the various forms of violence they may experience.