Pimps who abuse and force women into prostitution are criminals.
A woman who chooses to have sex with no strings attached in exchange for money by choice is not a criminal. I can't understand when one night stands are a norm nowadays but it gets all touchy when there's money involved and there's a person looking out for the other person. They aren't hurting anyone.
And you should read up on brothels OP. There was a BBC documentary on it where there was a madam talking about running a brothel in Soho. She ran it for about 8 years and it was known to the police but they left her alone purely because she was micromanaged on anything that came in. There was strictly no alcohol or drugs allowed. All girls had working contracts and were told to only work a certain number of hours. I see no harm in that.