Take sex work, for example. Legalizing it leads to an overall increase in sex trafficking. But it also does this: https://www.washingtonpost.com/news/wonk/wp/2014/07/17/when-...
My personal opinion is that if something is going to find a way to conduct itself in secret anyway (at high risk and cost) if it is banned, it is always better to just suck it up and permit it and regulate it in the open instead. Trafficked people are far easier to discover in an open market than a black one. Effects of anything (both positive and negative) are far easier to assess when the thing being assessed is legal.
Should we ban cash because it incentivizes mugging and pickpocketing and theft? (I've been the victim of pickpocketing. The most valuable thing they took was an irreplaceable military ID I carried (I was long since inactive)... Not the $25 in cash in my wallet at the time.) I mean, there would literally be far fewer muggings if no one carried cash. Is it thus the cash's "fault"?