That actually depends on the intent of the airplane ride:
18 U.S.C. Section 2423(b): Travel with intent to engage in illicit sexual conduct – U.S. citizens and residents are prohibited from traveling to a foreign country with the intent of engaging in sexual activities with a child. This is punishable by up to 30 years in prison.
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Ostensibly, that would not apply to those flying to Sexstein's island as it is part of the USVI and not a foreign country.
I bet when all the information comes out, those who flew there for underaged sex will appear the most innocent. Mark my words, they are going to find bodies there soon, probably hundreds of them.