Trafficking/exploitation is made worse by criminalization (which increases risks to people trying to escape it), and counteracted it sex work has the same workplace protective as other jobs (and even moreso if -- like many legal jobs with unique workplace problems) it has additional, job-specific workplace protections.

So that does the opposite of justifying prohibition.

legalization exacerbates trafficking: http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1986065

That paper has been criticised, e.g. by [1]:

doesn't measure trafficing, it only measures how many incidenses of trafficing are detected by the government. [...] This is important because we'd expect law enforcement detected cases of trafficking to go up under legalization, even if legalization reduced the actual number of trafficking victims. When prostitution is legal, victims of trafficing have less to fear by going to law enforcement (therefore making it more likely they will do so) and law enforcement can concentrate their efforts on prostitution operations that are still trying to avoid law enforcement, thus increasing their efficiency.

[1] https://www.reddit.com/r/PoliticalDiscussion/comments/49ivf2...