According to Bill Cosby, you can't really consider yourself to have been a parent if you only have one child because there is so much that's left out. For example, if something is broken, you know who did it. You don't have to go through an interrogation that would make Torquemada proud to discern the culprit. Nor do you have to endure the endless rounds of "He's touching me! He's touching me! Stop looking at me! He's on my side!" and the joy of competition for seats in the vehicle that comes as a part of not only every vacation, but every trip to the store.
As a father of four, I'd have to say I agree with this assessment, but I'd go one better and say you also need to have at least one boy among your brood. Otherwise you miss out on the joys of light sabers, dart guns, dragon slaying, pirate attacks, football, wrestling, and rescuing ladies fair. Every piece of furniture in our house has been climbed on and over, every bed has been jumped on (until Dad intervenes). In the evenings we are reading The Hobbit, and wondering what will happen to Bilbo tonight down in the goblin caverns under the Misty Mountains. (Hint: tonight we meet Gollum and find his "precious"!).
Don't get me wrong. Raising girls (I have two!) has joys all its own. But for sheer adventure, wild risk-taking, and a high probability of broken bones, what can compete with two boys?