Sex traffickers and pimps are for prostitution as gangs and the mafia are for drug sellers. We have legal marijuana without gang shootings, murder and smuggling, so the problem seems to be one about competition and making the legal options more competitive to the consumer compared to the illegal one. Preferable by making a self reinforcing environment where as more money getting put in the legal sector the more incentive the police has to go after the illegal one and the more the costumers want to spend at the legal one.