As House of the Dragon premiered, it has been brought to everybody’s attention that they have endogamic marriages. The House has proudly described how marriages always happen within their house.
Targaryens have been tying the knot to their own siblings, cousins, or other relatives for a long time. The very first Targaryen Westerosi King, Aegon the Conquerer had himself married his sisters and had children with each of them. Aenys I, Aegon’s son was his successor.
Even in the prequel series of Game of Thrones, we witness how King Viserys I Targaryen has married his cousin Aemma from House Arryn. His grandfather, King Jaehaerys I also had a successful marriage with his sister. There are only a few characters within the Throneworld who have married outside of their families.
For instance, King Viserys’ cousin Rhaenys Targaryen got married to Lord Corlys of House Velaryon. Both those houses have a very faint connection because of Aegon the Conquerer’s mother, Valaena Velaryon. Even Queen Aemma’s parents have no blood relation.
As per George RR Martin’s book series, The Song of Ice and Fire, the Targaryens married amongst themselves to keep the blood of the dragon pure. This is also one of the things which leads them to believe their superiority over other houses. As per them, the purity of their lineage is a mark of that.
This factor helps viewers, who were cringing when Jon Snow slept with his aunt, Daenerys in Game of Thrones, understand why they marry each other. The sort of inbreeding was happening constantly in their home continent before the Doom of Valyria.
Incest remains the most uncomfortable thing in Westeros even though the Targaryens try to normalize it at best. However, this did not come easy as this incest practice did cause a rift between Targaryens and the faith of the Seven kingdoms. This was caused after King Aenys’ children got married.
To settle this dispute, King Jaehaerys brought an order called the Doctrine of Exceptionalism. As per this doctrine, the Gods of the Kingdoms did not see it as a sin; as this made them different from others because of their dragons and Valyrian heritage.