From "The Sex Contract - The Evolution of Human Behavior" by
Helen E. Fisher:

	"Human breasts have no physiological function.  They are
subcutaneous deposits of fat around the teats and mammary glands that
can be cumbersome to the bearer.  They play no role in nursing.  In
fact, they can even smother the child.  Fatty buttocks don't seem to
have much use either.  They do store fat, and among African Bushmen,
where hunger may have been a problem, women have the biggest rear
ends in the world.  (These women's buttocks are, in fact, so large
that a small child can ride on his mother's back, standing with his
feet on her buttocks and his arms arond her neck.)  But fleshy
buttocks appear on all women, from the cave-dwelling Tasaday of the
Philippines to the disco dancers of Los Angeles, even though for most
of these women malnutrition is no problem.  Big breasts and buttocks
appear superfluous, as does the female's high voice and hairless chin
and chest."

	"Other anatomical features also evolved to entice mates.  As
Desmond Morris points out in "The Naked Ape", they all appear on the
front of the body--as if to encourage frontal copulation.  Fleshy
earlobes, protruding noses, everted red lips (which Morris says were
designed to mimic the genitals), and swelling breasts evolved as
sexual signals to invite copulation from the front.
	"Not everyone agrees with Desmond Morris.  They say that some of
these anatomical parts could have had other nonsexual functions as
well.  Perhaps.  But not female breasts.  These have no physiological
use whatsoever and biologists agree that their original function was
sexual invitation.  These sensitive, fleshy, delicate areas expand by
one third during intercourse.  The nipples harden at the slightest
touch, and for most women fondling of their breasts stimulates their
desire for intercourse.  Perhaps the breasts mimic the fleshy,
rounded buttocks that attracted males during rear-entry intercourse.
Whatever the case, protohominid males liked them in yesteryear. Those
with breasts had more young than those without.  And gradually
breasts became the norm.
	"But sex attractants evolved not only on the female.  Through
female choice, the males evolved them too.  Of all the primates the
human male has by far the largest penis--much larger even than that
of the gorilla, a primate three times a man's body bulk.  The width
of the normal penis provides extreme sexual pleasure to the female.
It distends the outer third of the vaginal canal during intercourse,
creating pleasurable tension, and because it is there the intense
contractions of orgasm are easily felt.  Furthermore, during inward
thrusts the increased width pulls on the muscles and other tissues of
the external female genitalia.  This motion stretches the clitoral
hood downward, creating gentle friciton on the clitoris itself.  It
seems that the largeness of this male anatomical part has no
practical function other than for sex, and undoubtedly it evolved in
size long ago because women liked men with large penises."