I think that people in Hong Kong are much more likely to judge a person's appearance based on a set of parameters, eg. 'She is ugly because she has freckles', or 'She is pretty because she has fair skin'.
I hear my mother do this all the time. She looks at a girl and almost seems to go through a mental checklist to decide if the girl is pretty or not. If the girl has dark skin, she almost automatically fails because in her mind, dark is not pretty, fair is.
The irony is, if my father had the same checklist in his mind, he probably wouldn't have chosen my mother
Why do we judge western tastes much more than Chinese tastes? I guess western tastes stand out more. Many HK guys are choosing non HK wives, but the mainland girls that they are choosing don't stand out as much as a Thai girl. Perhaps, local people wouldn't even consider a Thai girl as:-
1: They don't match the checklist,
2: They are concerned about the prejudice of others, or
3: They are prejudiced themselves (ie. I wouldn't choose her, she looks like a maid)
Most local guys will end up with a Chinese looking wife. It looks normal, and it doesn't really matter if the wife isn't pretty, or is a seelai, because it won't stand out.
On the other hand, westerners' choice of asian girlfriends will ALWAYS stand out, simply because the guy isn't Asian. Because of that, his wife or gf will ALWAYS be judged more. Even if his wife is the same seelai that the Chinese guy would have chosen, she will be judged on not being pretty, as if the western guy should find someone better looking. If she is hot, people will judge her as a slut.
I don't think that Westerners, on the other hand, are that quick to judge beauty in this way. It's certainly not based on the checklist that my mother and many other Chinese use.