IronGiant wrote:I think Desmond Morris once commented on this, saying that the top of the bum crack looks like a womans' cleavage (or was it the other way around ........ ? ) hence its attraction to men.
It's the other way around.
Basically, it's pretty fucking simple.
Before we evolved to stand on two legs, we came from species that walked around on four legs.
It's not so much the arse itself, as "the good omen for greater things to come" that it represents in such a quadrapedal pose. Surely I do not need to draw you the diagram?
In other words, "doggie style". The proverbial "beast with two backs".
There's an anatomical clue still in the modern human female's anatomy too: The woman's pelvic cage is slightly tilted. Furnishing easier rear entry in the quadrapedal pose.
There is also a secondary clue in that the small of the back is an erogenous zone, which would have been incidentally massaged during the act in full quadrapedal "mounted" position, increasingly the lady's pleasure at being humped mercilessly.
Another tertiary anatomical clue is that the female clitoris partially extends into the anal area. Suggesting either biologically endorsed natural contraception (but that would be a radical and wholly unproven theory) or simply that should the female decide to couple with an idiot who "misses" the right hole, there is mild compensation to at least not shock her into wholly abandoning the necessary survival strategy of sexual congress (though, trying to convince your girlfriend of the wisdom in exploring this anatomical curiosity is a surefire way of not getting any for a while, I report with due regret).
The "face to face" missionary is a modern evolution, as only humans (and a closely related ape species) are capable of this alternative. But, interestingly, surveys of "favourite sexual position" still chart "doggie style" highest for both males and females.
The female breast region is, indeed, suggested as being a simalcrum of the arse region, in an effort to ignite the male's instinct to those bygones quadrapedal days, whilst adopting the now preferred bipedal position.
Other anatomical curiosities unique to the human animal is that the human male has the largest penis to body size ratio of any animal on Earth. If one is aware of the basic concepts of Darwinian natural selection, this demonstrates that, for all the male bravado, women always get want they want in the end and is at least demonstrative, from the female perspective, of the advantages acrued from active deception in the act of coupling.
Which leads to another unique attribute: The human female animal is interestingly alone as the sole (known) animal that does not display external signals of being in heat. The poor male human animal is alone in facing the dilemma of having no pheromonal hints in determining the female's current state of horniness.
Interestingly, the common wisdom that women don't know what they want is actually scientifically accurate.
Scoring a set of cards containing photographs of various males at different periods in the menstrual cycle, the results showed a pattern that when out of heat, males who exhibited boyish and feminine appearance were preferred but when at peak of their cycle, unshaven brutish masculine appearance charted highest, sensibly and logically seeking "all man" (or nearest available close proximity to such an ideal).
Hence, DiCaprio might gain all the coos and tilted heads of affection out of season but it's Pitt or Clooney unshaven and rippling muscle that would be the preferred bedding partner.
The mistaken assumption which leads to the wisdom that women don't know what they want is to presume that what they want is a constant and fixed. It actually isn't and varies substantially according to oestrogen level. Or it could be alternatively termed that women know exactly what they want - remember, largest penis to body size ratio of any animal - at the prospective "peak" times but display greater flexibility in their orientation at other periods.
Why men have nipples, though, is a complete fucking mystery.

"Words ought to be a little wild - for they are assaults of thought on the unthinking."
- John Maynard Keynes