I've seen this explained on the discovery channel many times over the years; the 36-24-36 (or something like that) figure is generally considered to be that of a very healthy female in our psyche. It may or may not actually be true, but we are talking genetics here where we are genetically programmed to seek out the most healthy specimens to procreate with. This is as true for females as it is for males. Females are genetically programmed to find certain male features attractive as well.

Of course nurture plays a huge role as well, and we can be programmed by nurture to be attracted to the complete opposite.