I think it's clear that there is a biological reason for the male attraction to breasts.

Proof:
  1. Men can identify women by the look of their breasts, the curvature, the size and pattern of their nipples, and any scars. Men need only look at a womans breasts once or twice to be capable of identifing that woman.
  2. It's the most well defined physical feature that a woman possesses. Sure, lots of women have blue eyes, or blonde hair, short hair, long hair, a blue sweatshirt, but how many of them have DD breasts? Granted, any physical feature can be a mark-maker in the mind of a man, but breasts can't be faked, you have to physically modify them or do something painful or odd to them, like use a wonder bra.
  3. The vast majority of men have this attraction! You are never going to find a hetrosexual man that dislikes breasts! If you do, he's defective! Every man has a certain way he likes his girlfriends breasts to be, pouting, large, well rounded, each the same size, etc.