I reckon that even though sex work is legalised, and still caused issues, the problem is that there is no government regulation. It’s one thing to say by the government that they won’t prosecute sex workers, but if it’s not regulated and abuse still happens then nothing changed for all intents and purposes. Best analogy I could think of is like allowing food factories to manufacture food, of course. But if there is no regulatory watchdog to monitor and test to make sure food factories are not putting random and dangerous stuff into food, then legalising an activity is pointless.
Basically, the sex industry having been legalised by many countries is unofficially a libertarian set up. Yeah, the government exists and allow sexual transactions between agreeing parties, but they’re hands off on how the practitioners in the industry would conduct business. There is no government agency for sex workers to complain to if they’re abused. I know people would ask, how exactly would the government regulate sex? That, I will leave to policy experts.