Well you see, when a mommy point and a daddy point love each other very much, they like to hug. And they want to get as close to each other as possible. And the closest they can get is when the daddy point sticks his penis into the mommy point. It's kind of like scratching an itch. Or jumping rope--it's fun, but tiring, so you can't do it all day long. I forget how the rest of the book goes.
EDIT: Damnit, Smoke made the same joke I did; I did it better.