Doctrine of Exceptionalism
The Doctrine of Exceptionalism is the precept that King Jaehaerys I Targaryen worked out with the Faith of the Seven in order for them to tolerate the continued practice of incestuous marriages by House Targaryen. It was developed by King Jaehaerys, with help from Septon Oswyck and Septon Barth. It is called Exceptionalism for short, and its supporters Exceptionalists.
The Doctrine of Exceptionalism's basic tenet was simple: the Faith of the Seven had been born in Andalos of old, where the laws laid down by the Seven in the holy texts decreed that incest was an abomination. The Doctrine of Exceptionalism confirmed this, but with one caveat: the Targaryens were not like other men as they rode dragons and were the only ones in the world since the Doom of Valyria. In addition, they did not have their roots in Andalos, but in Valyria, where different laws and traditions held sway. The Targaryens wed brother to sister as the Valyrians had always done, and as the gods had made them this way, it was not for men to judge. The Seven Speakers preached the doctrine throughout Westeros. 
- Fire & Blood, A Time of Testing - The Realm Remade.