Thailand is my obvious first choice I'd recommend for someone taking a first trip. It is for sex, what Las Vegas is for partying. Everything is engineered to keep you safe, mostly out of trouble, and happy. there's no sublety or nuance in Thailand you need to worry about.
Two other countires worthy of putting on your roadmap (but not for beginners) are Laos and Cambodia. Similar to Thailand, but less crowded with mongers, same beautiful girls, and Laos is basically a little less than half the price of a sex vacation in thailand.
I'm really tuned into asian girls at this point, so while in the past i have mongered in many countries all over the world while working, i dont plan trips anywhere other than SE Asia. to each his own for that. to me its cheaper in SE asia, and the girls are prettier, younger, and more plentiful and accomodating.