Why did he want a boy? If he had a girl, she could have married a boy!

A king is always looking to give the throne to another king. Kings do business marriages to acquire land and wealth, while kings of other nations practically sell their daughters into these marriages for peaceful diplomatic affairs. If he had a daughter, she would have become someone else's queen, and their nation could acquire wealth (or nothing, depending on what he thought his daughter or her mother were worth). The last name is passed through the male, for the bloodline to continue. So for preservation of the royal family, a king must produce another king.