And then, yes, on top of those there is a whole conflicting mess of different desirable appearances which depend upon the period, culture, and person, driven by all the complexities of interacting human beings. But those factors can be boiled down to some fairly simple drives which have all emerged for one reason: to maximise reproductive success.
Which is all what you would expect from the result of selection pressures acting across tens of thousands of successful generations of descendants of ancient DNA-replicating machines.
Another way to say this is that it is in our nature to be highly responsive to environmental conditions, i.e. nurture. Nature builds the platform and tools needed for nurture to occur - but just because we are plastic in the face of nurture does not mean nurture negates or transcends the principles of nature (at least not so far, I believe.) If individuals had arisen in which this was so, they would have been selected against. You should read The Language Instinct, Steven Pinker makes this point about people who try and argue that the brain is effectively infinitely plastic and comes with no built-in "software" for processing and generating language.
(PS The main complication in trying to read human behaviour for evolutionary logic is that selection principles do not act upon individuals solely, because humans are all integrated into often multiple kin and common-identity networks of greater or lesser complexity. Evolutionary logic resolves at the level of these more-complicated networks.)
(PPS I believe that the head size-hip ratio theory is erroneous. Read http://blogs.smithsonianmag.com/hominids/2012/08/timing-of-c...)