Every city in the world has a red light district and some are far more obvious than others which are more hidden. Needless to say Bangkok had a history particularly during the Vietnam conflicts as a post for R&R: that being said today I don't think you are any more likely to run into a red light proposition in Bangkok than you are in London, Cologne, Vancouver, Sydney, or Buenos Aires. If you want to look for it you will find it- that being said I do find the area around Patpong to feel quite sleazy and thus would stay away from that zone unless there is some need to be there for work/etc.