The point to be made here is not that the sexualisation of the breast superceding the normal function of the breast is pretty modern. It's to point out that the perception of the female body, and the breast, is a cultural construct, and that culture changes regularly. A misconception always abounds when paintings such as these are highlighted: that women regularly fed their infants in public, as they are seen doing so in paintings. Nothing could be further from the truth.
Women weren't allowed in public spaces at all when these images were painted. Working class women, peasant women, would have been in the fields, but these were extensions of their domestic spheres, and roles as wives and daughters of peasant men. Equally, the lower class working women in the streets of towns and cities, were controlled and chaperoned by their men, unless they were powerful and rich widows, and even they had to follow rules. Upper class women were simply not in the streets at all. Women's bodies may have been naked and glorified in Art, but this was not a reflection that they were naked and glorified in day to day life. They were covered tip to toe in most places, most times. With the entire body being seen as something to be hidden, and the breast no more nor less 'hidden' that ankles, elbows and shoulders. Bare shoulders have actually been more scandalous in more cultures, more often, than the swell of the breast has been. Hair has usually been tightly controlled too, and some of these works are signalling domestic spheres by showing uncovered hair! Hussies!
Neither were these women nursing their babies at home, in front of guests. They were nursing them in their closed female quarters, if the household was rich enough, and negotiating their presence with male visitors in any event in smaller houses, nursing baby or not. Women were simply not free to be around male relatives and visitors in the home, without appropriate control and supervision. The addition of a baby didn't make any difference. Many women, in many periods, were not even nursing their babies themselves, as several complex systems of wet-nursing and hierarchy evolved in many areas at different time. The Queen sent her baby to the Duchess, the Duchess suckled the Queen's baby and sent hers to the Lady. The Lady suckled the Duchess's baby and sent hers to... and on downwards, 'till the farmer's wife was lucky enough to keep hers, and get someone else's too. Whilst the poor Queen got pregnant again.