Why is it that some people think it's inappropriate for children and teenagers to witness a woman breastfeeding her baby? Is it because they are secretly uncomfortable with seeing it themselves? Or perhaps these women (because it's only ever women that would dare to comment; a man would just avert his eyes if he was uncomfortable) did not have a successful breastfeeding experience themselves, and so observing a mother and child that have this bond just feeds their insecurities?
Whatever the reasoning, it is inappropriate to lecture a woman while she is breastfeeding about her breastfeeding. It causes stress to both the woman and her child, not to mention undermines the baby's need for nourishment.
A breast, when being used to feed a child is not a sexual object. Breastfeeding is not gross, lewd or sexual. And anyone who thinks otherwise has a seriously skewed notion of body image and function.