A major part of Thai tourism is prostitution, and a significant proportion of tourists visit to sample the local girls. Something like 1% of the female population of Thailand works in the sex business (most originating from the Isaan, a poor farming region in the North of Thailand, and in Bangkok to make their fortune), and it is almost impossible to avoid at least limited exposure to the business, whether it be from taxi drivers offering 'massage' (something to be avoided, as they will get a big commission), or the catcalls of 'hello handsome man' from female and katoey (ladyboy) prostitutes alike.
In particular walking through Patpong, with its night market, you will see literally dozens of gogo bars with their gaudy fronts and solicitous door girls leaving you in no doubt about what lies inside. Other well-known areas in the city are Nana Entertainment Plaza and Soi Cowboy (Soi 3), a street off the main thoroughfare Sukhumvit in the heart of the city.
- Understand is probably the right place for it. Southeast Asia would not be a good candidate, since it is too general a statement for all of SE Asia. It really is quite particular to Bangkok.