I'm not a lawyer, but I've read a couple articles about sex tourist groups being arrested that were going to Thailand for the purpose of having sex with girls that were legal in Thailand, but most likely underage in the US and Japan- the two countries from where the groups departed.

This was the work of some international task force designed to cut down on sex tourism in Thailand and elsewhere.  But it does, in fact, do exactly what the OP asked about.  It makes it illegal for an American to go to a foreign country for the purpose of having sex, even though it's legal in the country he will be visiting.

Now, presumably, they were going abroad to have sex with underage girls, but how can you arrest someone for that before an alleged crime has even occurred?  Hell, I think about banging teenage girls every time I go to the mall, but you can't be arrested for something you've only thought about but haven't done.

Even though I've never traveled abroad for purpose of sex (OK, my penis did accidentally fall into a few vaginas when in Korea, but I also had other business there too) I find this an interesting issue and will ask Legal Beagle about it.