Oh for FSM's sake!

The signs are very common in guest houses intended for farang. That would be you and me. As in, NOT Thai people. They would never stay in such a crazy place. Or visit you in one for that matter... unless it were for, ummm, "business". Hence the signs.

They are in no way racist, and to suggest so is not only ridiculous but displays a complete ignorance of Thai social conventions.

They are pretty much strictly for the benefit of farang to let them know that the operators of the guesthouse would rather not have it turn into some type of brothel or opium den. Sadly, that's what those crazy farang seem to do when carousing about in Bangkok... hook up with bar girls, prostitutes, smoke dope, and generally make a nuisance of themselves with no sense of propriety whatsoever.

Case in point: Thanon Kaosan

Fortunately Thais are on the whole very astute about such things and can differentiate easily between the social classes that exist in their country. You may have noticed that they don't tend to take things too literally, so neither should you.

Of course the signs also serve as a warning to let potential thieves, drug dealers and such know they are not welcome and in this sense they are there to protect you -- the "backpacking" farang ignorant of Thai social customs.

Ungrateful hooligans.