Why is the mixing of race and religion such a bad thing during the Medieval times?

   We saw our first form of mixing race and religion in the King of Tars when the white princess marries the Sultan, who is said to be a darker race. This marriage was of two different religions also. This happened to become an issue when the princess became pregnant and birthed a lump child. This was because she had not fully converted to the Sultan's religion because she did not want to so instead he converted to hers to try and fix their baby.

   Another time we see someone convert was in the Arabian Nights where the women converted to Muslim because that was her husband's religion and they end up having white children. In this story, there is no understanding of the Christian Faith which could have been one of the reasons it was easier to convert for the women in this story then for the princess in the King of Tars. 

  One of the biggest mixes of race and religion was in the Game of Thrones. You have Daenerys who is white and part of the House of Targaryen. Daenerys is given away by her brother to marry Kahl Drogo who is of darker skin color and is a part of the Dothraki. They end up getting married and Daenerys has supposedly converted to the Dothraki in ways, but she still holds on to a lot of her own race. She then becomes pregnant with Drogo's son who ends up dying when Daenerys has a lady perform black magic on him to keep him alive. After they burn Drogo, Daenerys goes into the  fire with the dragon eggs and does not burn up, but instead is found with the dragon's nursing off of her. She then takes both forms of the races and claims herself to be the mother of dragons. So why is interracial or religion marriages so bad? If people believed any of these stories of why mixing two different things together our culture would not be the way it has transformed into over the years.