This is false. When abroad, as a US citizen, you're still governed by US consent laws, not the foreign country's. The law states that whatever country you go to, it's a crime to have sexual relations with anyone under the age of 18.
That's the law.
Now, enforcing it, and prosecuting people for it is a bit harder, so that's the reason a lot of degenerates go to welcoming countries with lax laws and enforcement of said laws to have sex with underaged girls. However, if you're willing to produce and distribute evidence of the aforementioned crime