there is in fact only regulation as long as it is legal. how do you regulate if it is illegal? it only gets hidden then. and literally everywhere it went bad when it became illegal. everything you claim to want to achieve (regulation and workplace security) is completely lost and things get worse, more victims, less control, violence cannot be prosecuted cause none would go to police when anything happens, etc etc. , that is until it becomes legal again, but until then making it illegal even short time would cause way more damage than is possible to “fix” in a decade or two. just read about what happened where govs already took that path. if you want it to get out of control and destroy health and lifes, and create ground for forced prostitution (aka slavery), then yes, making it illegal is the way you get exactly that result.
and for the relationship thing… as far as i know (which is not much) the mayority of such customers already are in a relationship (mostly the one called marriage) while singles way less do such.