You can't go to a country where the age of consent is 12 for the purpose of having sex with a 12 year old.
That's a false equivalence. We're discussing state powers solely in the US.
What law would prevent states from prosecuting someone for this?
Article 3, Section. 2. The judicial Power shall extend to all Cases, in Law and Equity, arising under this Constitution, the Laws of the United States, and Treaties made, or which shall be made, under their Authority;—to all Cases affecting Ambassadors, other public Ministers and Consuls;—to all Cases of admiralty and maritime Jurisdiction;—to Controversies to which the United States shall be a Party;—to Controversies between two or more States;— between a State and Citizens of another State,—between Citizens of different States,—between Citizens of the same State claiming Lands under Grants of different States, and between a State, or the Citizens thereof, and foreign States, Citizens or Subjects.
That enumerates the judicial powers of the federal government, not of the states.
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