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.

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