Much of the western world has this strange perception that prostitution (in all its guises) in Thailand is a show put on particularly for the tourists, the farangs.
Sure, the glitzy bars that rancid_pickle describes with skill in patpong are for the foreign trade. However, Thailand has a massive prostitution industry, and the majority of it is for the locals. If you can bring yourself to believe the easily obtained figures, an estimated 75% of Thai men regularly visit prostitutes, and 44% of Thai men pay for their first sexual experience.
The US Secretary of State for Human Rights revealed that about 800,000 women under the age of 18 in Thailand were involved in prostitution (circa 1994). I'm not sure that sleazy tourists could account for this.....and how big is that number if you include over 18?
On average, the conditions for sex workers involved in the local trade are a tad bit worse than the girls who make it to the bars. Many of them are trafficked refugees from Burma, who come to Thailand in the expectation of quick money....many of them fully expecting to work in the sex trade....but not realizing the true nature of the plight they will find themselves in. Many of them come from Laos, same reason, same results.
Thailand also has a considerable domestic violence problem. There's most likely a link there. It's a big bad mixture of women and men who choose sex work as a viable means of securing a moneyed future, and those who are dragged unwillingly into miserable service of the needs of the uncaring.