TLC made a documentary that studies human mating rituals as if they were animals. It's called "The Human Animal," narrated by Desmond Morris. (Video purchase here:
http://shopping.discovery.com/stores...er=712281&gs=1) Anyways, it's a very interesting video and in it, it describes what traits typically attract the genders together and possible reasons why. Morris' theory about boobs is that they are related to nursing a baby, which relates to the primal urge to reproduce. Furthermore, to attract males, the boob-ness is replicated in the buttcheeks so there is a set of faux-boobs on the backside.
TLC usually plays it once a year. It's a *very* interesting show. Sheds a whole new light on the things people do. It made people watching for me much more interesting because Morris also explains body language.