Still not getting it. A side function is not the same as a primary function. If said massage shop was shuttered and then operated anyway you would have your argument. They do not however. They operate in full view as a functioning legitimate business. What someone does in the back room is harder to regulate than the business being open in the first place. It is either open or it’s closed. The back room business only happens when it’s open. Everyone knows when it’s open. Nobody shows when someone is in the back room when it’s open.Go to google.
Run a search for your city and "body rubs".
What do you think those businesses are selling ?
I'll give you a hint, it's not back rubs and foot massages.