It's not the only rule that can complicate life. Later Russian succession law required that the marriage be equal -- the bride or bridegroom had to also come from a sovereign house.
On the other hand, heirs can also spring from -- less regular unions. Random couplings generally don't cut it, as there is the little question of whether the baby really is the man's. But concubines often hit the standard, even if her sons' claim is inferior to that of the wife's. When the Church was moving into the marriage regulation business, there was to-do about how Jacob's concubines' sons had been his heirs right alongside the wives' sons. To be sure, while some were demoted to mistresses, others were promoted to wives. . . .
But before then, there was a long time when a "natural" son might succeed as well as his legitimate brother. Especially in Scandinavian regions where the practice was to lead out all the sons after their father's death and see which one the subjects hailed as their king.