While the concept of royalty might seem different and removed from the rest of us, the biological process of reproduction remains the same for everyone.
It's important to remember that while kings and queens were often portrayed as divine figures or symbols of power, they were still human beings with the same basic needs and desires as anyone else.
There might be some specific historical details surrounding royal marriages and the role of lineage, but at its core, the answer is as simple as it is for everyone else.