This is a circular logic similar to the rhetoric against marijuana decriminalization. In both cases we see references to concurrent illicit activity. Association with gangs. And an appeal to ‘protect the children’.
But the truth is that in a decriminalization environment we see revenues kept out of gangs hands. It is also salient to point out that in the case of pot decriminalization we are lessening the exposure to harder drugs. A dispensary will only be dispensing pot. Likewise decriminalized sex work will remove the stigma for the sex workers. When someone no longer thinks they are a ‘criminal’ they are less likely to engage in other illicit acts (like drugs).
Also legal sex work will not mean legal sex work for children. The availability of safe, well lighted places for sex work will pull the rug out from under the market for victimized children. And free up law enforcement to target predators who coerce children into sex work.
I encourage you to view these subjects in terms of harm reduction as opposed to the lens of morality.