Christopher Ryan, who blogs about "the evolutionary origins of modern sexuality" tries to explain why straight men are fascinated by "what are, after all, small bags of fat":
Theories range from the belief that breasts serve as signaling devices announcing fertility and fat deposits sufficient to withstand the rigors of pregnancy and breastfeeding to "genital echo theory": females developed pendulous breasts around the time hominids began walking upright in order to provoke the excitation males formerly felt when gazing at the fatty deposits on the buttocks.
The Dish 