Since this is the Book Talk thread:
Raeghar didn't care about legalities. He was obsessed with the prophecy about TPtwP, whom he believed to be Aegon, his son with Elia. But the prophecy also speaks of the dragon having three heads, so he had to father another child.
Unfortunately, Elia was weak and sickly, and was unlikely to bear another child, so Raeghar chose Lyanna.