Would you happen to know much about the animosity Koreans have to halvies?
I've done a little bit of reading on it. I would summarise it like this:
Modern Koreans typically believe that national culture and pride are more or less a matter of bloodlines, and by bloodlines they mean pure bloodlines. You may be familiar with the national creation myth, which basically asserts that the great Father Tan'gun established the Korean nation some 5,000 years ago, and since then, the country has maintained a pure bloodline (i.e. not "mixed" with other "races"), a pure language, a pure culture &c.. It's a myth, but many Koreans still have a large degree of faith in the idea that there has been a Korean "race" which has been maintained for five millennia.
In actual fact, this myth only became entrenched last century, under Japanese colonial oppression. Both during the Japanese annexation (1910-45) and immediately afterwards, and especially after the Korean War (1950-53), Chosŏn/Korea were poor countries with nothing to boast of. The political system was dysfunctional, there was no food self-sufficiency, and no substantial material achievement with which Koreans could face the world with pride. Instead they suffered continuous threat from superpowers in a dynamic geopolitical region. During the Japanese colonisation, the independence movement leaders consciously sought to create a sense of national unity and pride and they did this by propagating the mythical ideas of national unity and purity as described above.
There is a real sense in which this sense of national, linguistic, cultural and biological purity played a large part in pulling South Korea through to its current economic strength, and in keeping North Korea insular and submissive to a "father" figure. The same colonial experience produced different results in different circumstances, but you can nonetheless trace both the North and South Korean national identities back to this myth of purity. As an aside, this myth of purity is part of the reason why South Korea is having such a hard time with the idea of multiculturalism.
We cannot overemphasise the fact that the purity myths are complete myths; nothing more, nothing less. In the Chosŏn period, before Japanese annexation, the bureaucracy had a process whereby foreigners from Japan and China could be completely absorbed into the population. If a foreigner wanted to live in Chosŏn, they were not immediately accepted, but by abiding by the right regulations, then his grandchildren would be given the opportunity to become complete Koreans. They would have all the opportunities that they would have had if their grandparent had been a Korean, and they would also have all the same responsibilities as a Korean (military service etc.).
And ever since 1945, there have been countless children born to Koreans and U.S. soldiers. Despite all of this, so many Koreans still think one has to look a certain way to be a "true" Korean, i.e. display the typical biological characteristics, speak the language and understand the culture etc. It's because of the purity myths.
Of course, a lot is changing, but most Koreans remain uneducated about the truth of their own history. It's not entirely incomprehensible, because colonisation and the war were such devastating experiences. They provoke violently emotional reactions in people, which hinders their ability to learn the history in full.
Sorry that was long, but maybe it helped.