[Nyororin]

In my observance, if we ignore the cultural opinion of a certain group by another... a culture transplant issue strongly comes into play.

I find this something that really stands out as a huge roadblock for western women and Asian men in particular. Asian women tend to have a huge following with western men for many reasons both true and incredibly stereotypish... But on the other hand Asian men tend to have the opposite - for both true and stereotype reasons. The same features that are deemed attractive in Asian women are negative traits in Asian men. (ie. smaller stature, cultural background where women are more subservient, etc)

In a relationship, there tends to be the expectation of the woman entering the man`s culture. In marriages it is far far more common for the couple to move to the country of the husband than the wife - largely for financial and employment issues. A woman from an Asian country is far more likely to be positive toward a move to a foreign husband`s country (as culture in Asia tends to lean toward men being the head of the household and the woman following) than a western woman.

Add into this the fact that Asian men tend to know the prevailing cultural opinion, and the widespread belief that "western" culture and society is superior... And it becomes (particularly for Asian men and western women) quite hard to imagine a western woman transplanting herself into an "inferior" society.
An Asian woman is usually influenced to feel that a western husband is a "step up", so to speak, and a western man is usually influenced to feel that an Asian wife is a status symbol. An Asian man is influenced to think that no western woman would be interested to begin with, and western women are influenced to think that Asian men and culture is inferior.

Of course this is all just my opinion, and of course there are many relationships in which these factors don`t really come into play at all. But at the same time, cultural ideals do have a very strong effect on what people actually do even if they don`t personally feel that way. As the OP said, it`s fairly rare to see an Asian man - Non-Asian woman couple. That alone is a stigma. It stands out. There is pressure there, and most of it negative. No matter how the individual feels, pressure is pressure. Stress is stress. So it just becomes something that isn`t done.