Goryeo/Joseon era

Yes, it is incest, but like many royal families in history, it was normal for them to marry. Back then, they had no knowledge of genetic mutations that would cause health problems for their offspring.

If you Google it, you will find countless examples of royalty everywhere that married their relatives. Examples: Cleopatra was actually married to her brother. Queen Victoria married her kids all over Europe to cpusins and spread the hemophelia gene. One of Span's kings was so deformed by incest that he couldn't walk or eat properly. Ex. 

Royalty throughout history did this for two reasons. One, they believed it kept their bloodlines pure and two, they did so for political reasons.

I think if you actually look into it there was a time during the Goryeo reign where princesses from Mongul kingdom were sent to Goryeo to marry the king there so that there was some Mongul bloodline instead, since they wanted to keep some control over the Goryeo line. 

 colay17:

Thank you all. I suppose marriage then was not just because of love. It may be true still today for some culture although not quite for purity of bloodline but more for economic reasons. 

 Mine eps 1 mentioned about this. Married for wealth family is about business.