The reason they do it is because it feels good and they reely reely reely wanna do it ! The reason that system developed the way it did is simple:. The ones that wanted to do it did it. They had more children. Their children inherited the desire to do it from their parents. The ones that didn't want to do it didn't do it. They had fewer children. And the children they had were not so interested in it. After a while you can see why many many more wanted to do it then the number that weren't interested.
