Well, let's start with the social thing. In a sociological study of 119 distinct cultures and sub-culture groups, only in 13 were breasts considered sexual. In only 11 were the breasts touched erotically during sex. So, in large part, attraction to breasts is a cultural phenomenon. This can be compared to the practice of
foot binding in China, that was considered erotic. Essentially, people can be trained to sexualize any part of the body.
Now as to why men act stupid around breasts. That is actually a simpler matter to explain. Simply put, men act dumber around breasts because men
are dumber around breasts. Libido in humans is governed by testosterone. Men have, on average, ten times more testosterone than women. Hormone levels are non-linear, meaning that men having ten times the testosterone of women does not mean they have ten times the libido of women. It's actually a much greater difference than that. Men think of sex once every six seconds, on average. That means a man, in the course of an hour, can think about sex more times than a woman will in the course of a week. In my experience, women's reaction to such information is "how do you get anything else done?" (As an aside, the answer to that is we do other stuff
while thinking about sex. Why do you think everything men build looks vaugley like a penis?) Men's reaction is usually, "That sounds about right," or, if under the age of 25 and at peak testosterone levels, "Really, that little?" When men become sexually aroused, more testosterone is released into a man's system. In our culture, breasts sexually arouse men. Testosterone [url=testosterone]impairs the higher brain functions.[/url] Thus, men actually suffer from reduced cognition when aroused. Just ask any man. It is usually referred to as the sense that one's brain switches off and, "My little head takes over."
Hopefully I've answered your question. Breasts sexually arouse men in our culture. The arousal increases testosterone levels. Testosterone impairs the cognition of the man. Thus men in our culture become dumber around breasts.