what is that?

I think he's repeating the urban myth of American soldiers getting "black syphilis" and not being allowed home.

sex tourism. However the number in Chiang Mai is likely minuscule to those in Pattaya, or the "poor" (and thus can't afford to live in Pattaya or Chiang Mai) in Isan (an extremely poor part of the country well known for daughters being "sold off" to foreign husbands 2x or 3x their age)