I've been to Thailand a number of times on military missions and I've seen these huge metal signs, about 5' wide and 6' high, warning Americans that the US government will prosecute anyone found to engage in sexual relations with underage children. As for how they can prosecute, it's probably in the fine print of your passport application, although I'm just speculating. Surely there's at least one FReeper who is a legal beagle.
As posted in post #8 there is a US law against going somewhere to have sex with minors. Kinda sick that we *need* a law like that, but clearly we did. What happens in Thailand doesn’t stay in Thailand.