Or rather, their family would never allow them to marry foreigners or non-Koreans because of the multitude of cultural rites/family customs the first-son (and his wife) has to perform.

Can a South Korean marry a foreigner?

Foreigner marrying a citizen from South Korea A foreigner who wants to marry a South Korean citizen must produce the following documents: Evidence of identity and nationality - both birth certificate and passport. Affidavit of Eligibility for Marriage - issued and notarised by the consulate of the home country.