While I disagree with your headcannon of Queen Serenity being Sailor Cosmos, I do share the belief that Queen Serenity asked a greater power to give her an heir-immaculate conception if you will. However, the idea that there may have been a King Serenity at some point doesn't seem that far off either (Though it would still be Queen Serenity being the head ruler.) It all would depend on just how different the people of the moon were from earthlings. They are, in essence, not human. It's possible that breeding was different-but also possible that it was the same.