One tour called 'Sex in the City' and led by journalist Ulrich Mattner allows visitors in the German city of Frankfurt to learn more about the red light district. But there's a catch - the tour is for women only.

Mattner's connections to people working in the red light district make him the best man for the job. He takes his female tourists to visit different sex workers, from dominatrix specialists to strip bar managers. Sometimes his tour participants even get to see Mattner tied and whipped like a customer.