Board Thread:General Discussion/@comment-26923997-20160820130303/@comment-27751431-20160821073412

Another factor was that Targaryens lacked dragons to enforce the rule. They would be more inclined to listen to the Faith or the great lords of the realm.

Maegor the Cruel has multiple wives, from lines outside his own, so there was and is precedent. However, the extent to which the Targaryen kings could defy convention, the Faith, and the opinions of the other lords decreased markedly after they no longer had dragons. If you have a dragon, you can have as many wives as you want, and people are less likely to object.- Quote from GRRM