Legalized prostitution, as in the Netherlands and Germany, “is an open door for traffickers,” claims Janice Raymond, an activist with the U.S.-based Coalition Against Trafficking in Women.

On Saturday nights, as many as 300 young women line the margins of E55, a Czech highway near the German border. Their costumes vary: light frocks, skimpy red dresses, glow-in-the-dark Spandex pants. They speak a babel of languages: Czech, Romanian, Bulgarian, Hungarian, German. But they have only one thing to…