I have seen plenty of warnings that it is a federal crime for a U.S. citizen to pay for sex with, or buy, possess, or even view "pornography" of, any person under the age of 18, anywhere in the world.

Which is to say - it may be legal for a U.S. citizen to have sexual relations with a 12 year old in another country, but not if they are a prostitute.

Like all restraints on trade, problematic and counterproductive - though at least the claims that such exchanges are abusive may in some cases have merit, unlike the truly bizarre claims that viewing a photograph necessarily causes harm. At least it doesn't require magic to work.