Sometimes, things must be done not for historical realism, but for game balance. Royals cannot be banished from a realm and that is a powerful perk. Royalty generally requires enough realm support to be elected or win a rebellion, and maintaining that support for a long enough period of time. The only exception would be a duke seceding to form a new realm, but even they could lose Royal status if their new realm doesn't support them as ruler (its happened before). So I don't think its a good idea, for game balance reasons, to allow a ruler to single-handedly grant Royalty to a roleplayed spouse, as Royal status should be something the realm gets a larger say in.
