There’s an impression amongst some in the ASOIAF fandom that House Targaryen has a history of polygamy, and that Westeros permitted this. Ergo, Rhaegar could TOTALLY take Lyanna Stark as a second wife and thus Jon Snow is legitimate.
As I’ve ranted elsewhere, Jon does not need to be legitimate. There’s just no need to tie ourselves up in knots to find a way for Jon Snow to be Jon Targaryen. Or Jaehaerys or Aemon or Visenya/Viserys or whatever Targaryen. The world will be saved by a bastard son of House Stark… and that’s fine.
But the need for a legitimate Jon, and this idea that House Targaryen was super special and never, ever subject to the social norms, customs and laws of Westeros when they conquered and took on the role of the defender of the Faith seems to perpetuate throughout the fandom. This post is about breaking down how and why we can be reasonably sure that the Targaryens took on the laws and lores of their new Seven Kingdoms. And, bonus…. polygamy was never “common” in Valyria. It was just a thing that happened.