It's trying to protect kids in foreign countries from Americans travelling overseas explicitly to hurt them. It's a good thing to do.
This won't do that honestly... Unless they are stopped from even entering those countries, it simply won't do it because many of those countries tend to either not have the infrastructure to track and prevent or the progress needed enough to care. Putting this stamp on someone who goes to India or some other poverty stricken country with backwards views on woman/girls will not stop them from doing so.
The better way would be to track them the same way we do terrorists (which we already do, just not on the same scale internationally), labeling it on their passport only serves to bring public opinion into the matter. Also just like potential terrorists, going to said countries that are full of sex trafficking is not enough to convict them of any wrongdoing.
I simply see no reason why or what good this policy would do other than public shame.