I never said she reigned, but she was indeed the rightful heir until a son appeared. In that example, I think it's also because she lacks the capability, he implies that she could have been a choice had she been strong.
Most of all, though, I think the fact that Robert was so keen to have both Viserys and Daenerys murdered says a lot. They also kill both of Rhaegar's children, the daughter as well, though were not instructed to kill their mother, so if daughters didn't matter then I don't think they would have killed Rhaenys as well, but who knows.