I think you are overthinking it too much.
So he went to Amsterdam on a mate's holiday. That was straightforward.
The top activities to do in Amsterdam when you're with your friends are to visit the museums, explore (but not necessarily do) the 'shops of a high' nature, and visit De Wallen for the touristy windows of workers.
Not everyone does, and it depends on the type of mates who are interested in this.
You expressed concern about him going to this area of Amsterdam, given you don't feel comfortable.
But if you flip the switch, and you go on a girl's holiday to Ibiza, Magaluf, Aya Napa etc, I'm pretty sure your boyfriend would feel uncomfortable about you going to clubs and being around all that sexual energy too.
Have you yourself been to the district? Honest question.
For my advice on what you should do:
A) Suggest you both go to the district together on a future visit, join a walking tour that explains the history and why it's even called the district, and use it as an educational visit, rather than a sexual release thing.
B) Have a chat, say it made you uncomfortable that he went, and find a way past it. Ask him about his thoughts if you went on a girl's holiday to see Magic Mike or some other holiday that had sexual areas in it.
C) Stop overthinking, allow him to enjoy his holiday, check in every so often and enjoy your days with your friends. What you don't want, is him to return with a whole textbook of activities he and his mates did (sightseeing, museums etc), and you just sat on the sofa, twiddling your thumbs, overthinking everything. Get out there, meet your friends, do activities you enjoy, make the most of the week whilst he's away. So that when he returns, you can tell him about what you got up to.
Hope this helps.