Typically gormless moral minority
It stands to reason that if you make activities like running a brothel illegal then, surprise surprise, it'll attract criminal elements who'll go to any lengths to make money out of it. Criminals love people trafficking because it's not regarded, for some bizarre reason, as being as serious as trafficking drugs so, even if you're caught, the penalties are less severe. So, what do they do? They combine people trafficking and brothel keeping because it's a nice, big, low risk earner.
The sensible thing to do, given that prostitution is currently actually legal, is to licence brothels run by prostitutes themselves. They could then have the protection of the police, regular medical checks and the unmitigated joy of paying taxes. The majority of men will then frequent the legal brothels and the illegal ones will die a death, reducing/eliminating the attraction of sex trafficking and the social harm of the activity.
Legalise most drugs too while we're at it and we'll increase tax revenue, starve criminals of income and free up some space in our prisons for MPs.