I propose that the female ass replace the rack as the sexual dimorphic characteristic that men lust after. The female boobs are too functional,subject to shape shift and loss,hard to get to,and not generally as firm as the ass.
Take these young ladies from Africa for instance:
They breast feed their children and manually grind their flour. Their boobs are far from perky spheres by the time they reach adulthood (of course they have other issues that I may broach at a later date: The use of patriarchal customary laws constrained women’s access to resources,The legal framework prohibited poor women from entering informal labour markets,Prevailing traditional gender biases exist against women in the allocation of land,Women’s access to resources is limited due to biases resulting from the family arbitration system,Women have no access to legal aid and are severely constrained in seeking remedies from civil courts,Lack of women’s grassroots organisations prevents their participation in development activities). Piqued your interest? http://www.ethioembassy.org.uk/fact%20file/a-z/women-1.htm
Alas, I digress (I probably have ADD--where is my Ritalin)!
Most women after childbirth and breastfeeding and generally reaching ages above 19 will not have perky breasts. It is nearly impossible to get boobs to regain their former turgidity and bounce. If women have had breast cancer, the breast is either partially or totally removed and she needs surgery. The only women who have magnificent specimens after their 20's have spent $1,000's on surgery and may have to repeat a surgery and/or lift again every decade or so! No thanks!
Enter the ass cheek: the glutes generally retain their shape and firmness after childbirth and breastfeeding (although the hip width may change due to the pelvis physically separating). Plus, it is very easy and inexpensive to keep the ass and get it in shape!
Boobs are difficult to get to: there is the formidable bra that can be difficult to unhook or downright impossible to get a sports bra off easily! Not to mention the bothersome need for eye contact and/or conversation.
Now with modern clothing trends, the ass cleavage is easily viewed (dime slot) and can be readily caressed without many calories burned. Panties may provide some hindrance, but there aren't any hooks or complicated discard methods. Not to mention it is much closer the real object of men's desire: the Vagina (play celestial music now).
Boobs are not as firm as the ass (pre-surgery)! Can you slap a boob and hear a resounding smack? Will the boobs resist the pressure of weight? Can you grab on tight to boobs without your partner screaming in agony? No, butt you can smack an ass cheek (http://churchofjessicabielsass.blogspot.com/, http://spankcheeks.blogspot.com/), put your weight against it, and hold on tight!
The moral of this tail: Ass cleavage has resilience and staying power without the tedious functional jobs related to child rearing! Okay now, lactating Nellies get your panties in a bunch!