Back in the day, if you were a king, or a ruler of any kind, the idea of marrying your children off to the offspring of another kingdom was a common practice. That’s how alliances were formed. If you were worried about country “A” invading you, you could marry your daughter off to the prince of country “A”. A show of kinship. Attacking your in-laws was frowned upon (that was the theory, but it didn’t always work out that way in practice). Also, countries "B" and "C" might think twice about invading you. This intermixing was so commonplace in European history that no one thought twice about it. There was also the prejudice that only someone of noble blood was worthy of another with noble blood. Royal children were pawns in a royal chess game. Take the case of Lucrezia Borgia. She was royal in some sense of the word since her father was the Pope (Alexander VI). We all know that priests and popes were not supposed to have children, but such things were winked at in...