LOL I've been to the dam like 7 times, if my GF told me she's dump me for going, i'd dump her!
there's more amsterdam then hookers, if you don't trust that he won't have sex, you shouldn't be in a relationship with him anyway...
stop being such a pain in the arse (you can gurantee his mates won;t like you, if you stop him from going, may result in some resentment from him)