Evolutionarily speaking, humans used to walk on all fours like our other primate cousins. There are a number of reasons humans adapted and evolved to walk upright. When our early ancestors lived on the open grasslands of Africa, it was hard to see predators approaching over the horizon being on all fours. Therefore, the pressure was to either walk upright to survive, or face death at the mouth of a large predator cat.
Now, when human females walked on all fours, the protruding butt was the major attraction to males when it came to courtship and mating. However, when humans begin to walk upright, this feature became less prominent, thereby causing males to not want to mate with those females. It's believed that large breasts developed in humans as an adaptation to this dillema. If you look closely, the shape of a woman's breast coupled with the cleavage mirrors the primeval human butt when we walked on all fours. Therefore, males are genetically inclined to be attracted to breasts because it reminds them of the butt, which reminds them of mating--i.e., sex.
