I think you're missing the point. There appears to be no aboveboard legal prostitution in Sweden, nor is that what the law envisions. Rather all prostitution involves someone committing a crime (the John), and the law has chosen to not directly punish one party because that leads to undesirable outcomes in cases where that party wasn't willing participating.

There is no trap, if you don't break the law, and don't help other people break the law, then you aren't involved in the industry at all.

I'm not going to take a position on whether or not prostitution should be legal, but pretending that prostitution is currently completely legal when it always involves a crime doesn't seem like a good argument.