This goes back to the early cave men where women with wide hips have the probability of giving birth "easily" compared to other women. In an era without any advanced medical knowledge, this gives them a good chance of surviving the birth together with the offspring, hence they were more desirable in the eyes of the men. This was hardwired in the brains of the male cave men and till this day, where human has made major leaps in advanced medicine, our brains didn't evolve enough to overlook this factor. Plus, like the other guy has said, it bounces when we hit it from behind.