Also, I don't really know what the law is in the US! Is it like it is in Canada (technically legal, but functionally criminalized), or is it flat out illegal. Am I correct in assuming it's a state-level law? If yes, how much variation is there from state to state?
State-level and in some cases city-level (it's legal in Reno, technically illegal in Vegas though the "escort" loophole is applied pretty liberally -- I think; one of our Vegas rats can correct me if I'm wrong). Entirely illegal in most of the country.
Are we laboring under the impression that the child trafficking market doesn't exist because the Russian Mafia is too busy with the adult trafficking market here?
Right, that's kinda where I am WRT Zara's argument. Sex trafficking will and should continue to be illegal under any scenario where consenting-adults prostitution is legal; I don't see the latter increasing demand or tolerance for the former.
The washing-machine analogy is a little bizarre, but the porn one is pretty apt (and indeed fits under the same "sex worker" umbrella). Mainstream availability of porn has not created a mainstream acceptance of child porn.
I suppose Zara's argument DOES hit on the "gray area" in this example -- while every professional porn distributor keeps records and ensures the ages of its girls, there's been a proliferation of amateur porn that doesn't have any documentation with it. It's a lot harder to verify a model's age than it was when the record-keeping law was passed following the Traci Lords scandal.
The equivalent for prostitution would, presumably, be small-time pimps operating outside of the regulatory system. And yes, they would still exist even if prostitution were legal, and no, they wouldn't be terribly concerned about age verification.
The argument in favor of legalization, then, is that situations like that would not increase -- indeed, I think people arguing for it argue that they would decrease, precisely because women (the ones of legal age, at least) would have a better option. Would you rather have a pimp, or a union?