The reason is that clients ask for unsafe sex, and due to competition, the women are more willing to oblige.
De Rode Draad says that this is also caused by the increasing number of foreign (East-European) women in the Red Light District.

The Rode Draad also says that women get increasingly perverted requests, such as dirty sex and strangulation sex.

The Amsterdam health authorities confirm they too received 'signals' that unsafe sex is on the increase, although they can not produce exact figures. They can not confirm the increase is caused by the increasing number of foreign women in the RLD.