You see an issue crop up in many areas where something is generally found distasteful by society at large, so it is made illegal by various means, leading to greater harm than if it was just legal and regulated. You will not eliminate demand, therefore you will not eliminate those attempting to supply the demand. If sex work was legalized and regulated, it would be much safer for the workers, customers, and even those who choose not to engage with it all. The current legal environment creates space for trafficking/slavery, public health concerns, and a lack of physical safety for all. While a legal and regulated industry won't completely remove the black market, it would greatly reduce it and the harms it causes. Is there still demand for dusty garage tattoos? Yes, but that is certainly not the majority of the market.