"There's no way these biases are as entrenched in the general population as they are in sex tourism centers like Pattaya and Phuket"
Fallacious reasoning. You're missing the key confound in your logic which is that it is mostly in these places where Thais will come into direct contact with these people so they are just as liable to form these impressions on their own without any prompting from Caucasians as you seem to suggest. I spend a lot of time in Singapore where there is a large South Asian worker population and only a relative handful of Caucasians, and the girls here (local, Filipina, Thai, Indonesian etc) will all talk about some of these issues re Indians (and not all Indian, mind you). So these 'stereotypes' are more the product of experience than the 'evil influence' of Westerners, as you would like to think.