Humans are animals... we evolved with instinctual desires that affect our behavior, same as all the other animals.
It is the same in most animals that reproduce sexually (and even more true for mammals).
Females who (appear to) have what it takes to reproduce more successfully are more desirable to males. Males that are stronger, and have more control of resources (this means money in human terms) to provide for children are more desirable to females.
In our attempts to be "civilized", we often run away from the fact that we are affected by impulses that evolved over millions of years.
Agreed, we are animals. And yes, we are affected by ancient biological imperatives. I'm not denying that. But what I am saying is there are exceptions to this so-called "rule", myself being one.
I don't particularly find big hipped women to be more attractive than smaller hipped ones. And I don't think I'm alone on this. This may be in part because I realize cognitively that big hips don't necessarily equate to better child bearing, any more than smaller ones equate to worse child bearing. My first wife had hips so wide, she could barely squeeze her ass thru a standard sized door. But both of the children she bore required C-sections. My second wife's hips are so narrow she could fall down the shower drain, and yet she bore our child naturally. So, unless all of us men who like thin, small breasted, small hipped women are suffering from some sort of biological anomaly, I don't think the rule holds any water.