That's simply not the case where prostitution has been legallized.
I understand why people oppose prostitution due to moral objections (although I disagree, looking at it on a strict cost/benefit analysis to society), but the statement you made is really no different than when the anti-gunners claim that concealed carry laws will have "blook running in the streets," or shoot-outs over road rage. It's just not happening. And where prostitution is legalized, it's a high dollar business, and spreading disease would jeapordize the profits. And that's something that nobody would tolerate.
Mark
The increase in the spread of STDs by legalizing prositution do not come from the legal sex industry itself but from its illegal periphery that would continue to exist but driven to offer what is still illegal - unprotected, sadistic/masochistic, underage, sodomizing, etc.
This periphery will increase as the industry itself sates the illicit yearnings of stupid and naive men who should know better.
Keep prostitution illegal for ever or let evolution take its course with the naive and innocent lured into acts of stupidity.