The point about Thailand is that there is a very open sex trade, dumb-ass. You can't fail to notice it if you spend any amount of time in the country. It's no different in that respect from people choosing to visit Amsterdam to buy weed in a coffee shop rather than from a man on a dark corner.
The Western-focused large-scale commercial sex industry makes efforts to keep itself 18+ (id checks, etc.), because there's far more money in attracting Western men on a large scale to have sex with adult women than being an international haven for child sex tourists.
That's what drove the likes of Gary Glitter to Cambodia, because it's a laxer jurisdiction.
There are half-a-million child sex workers in Brazil, and clearly anyone who actively seeks to have sex with a child can do so in Brazil, as no doubt in Thailand and many other countries. However unlike Thailand, Brazil does not have millions of excess male tourists, and people don't assume that tourists to Brazil are going there to abuse children.
The point is that the reason Thailand has such a large sex tourism industry is because it has brightly lit go-go bars with adult prostitutes available. No doubt if you asked your tuktuk driver he could find you underage prostitutes, but the same would be true in Cambodia, the Philippines, Vietnam, Indonesia, and so on, and probably in those countries you are less likely to find yourself in serious legal trouble.
For example, the sex industry on Batam, in Indonesia, just off the coast of Singapore, serves Singaporean men, and there are many trafficked children there, and with neither Singaporean clients nor the Indonesians typically seeing a problem with men wanting 'younger' prostitutes, there is no real restraint on underage sex there at all.
E.g., take this story berita-medan.blogspot.co.uk/2010/03/anak-sd-dipaksa-kawin-sama-kakek-60.html it describes a girl of 12 being married off by her parents to a 60 year old man, in return for a house, motorbike, and cash.
Same story as Thailand: www.bangkokpost.com/news/local/330397/daughter-forced-to-marry-for-b2000 but Indonesia doesn't carry the association because it doesn't have the gaudy go-go bars.