It has become a real problem for Canadians visiting the States[1]. The US government doesn't seem to care whether it's legal in Canada or not. Presumably that's because the law is written such that foreign legality is irrelevant.
I'd hope they would apply this rule to people who engage in sex tourism with children, legal in that nation or not for example. At the federal level the crime of cannabis distribution is on the same level as that.
It does already - I looked it up. It even applies to interstate lines with varied age of consent - although it is a bit more intent based and grayer.
For example - going from say California - an 18 age of consent state to sleep with a 16 year old in Nevada is illegal sex tourism. Likewise bringing a 16 year old from California to Nevada to try to evade the age of consent is illegal.
But going to Nevada from California and sleeping with a 16 year old while not there for that purpose (say on a business trip) would be legal.
[1]: https://www.thestar.com/vancouver/2018/07/05/canadian-cannab...