It's a hangover from the puritans of the Victorian era. At that time almost any part of the female anatomy, was considered offensive and immodest. It was even impolite to refer to a woman's legs; they were called limbs. And it was at that time that "modesty panels" were added to desks to that women's legs would not be exposed to people passing by,
Some people are mired in the past and still see the naked human body -- male or female -- as a source of shame.
We're growing up, of course, but in places like the American Bible Belt and the Islamic theocracies too many are still wallowing in the past.
Breast feeding is natural and non-sexual.