You've hit the nail on the head - breastfeeding is entangled with our relationship with nudity. I don't personally condone eating nude in public in a restauran

You've hit the nail on the head - breastfeeding is entangled with our relationship with nudity. I don't personally condone eating nude in public in a restaurant, but there is a broad spectrum of ambivalence towards nudity. For instance, plenty of europe is fine with mixed sex nude saunas and beaches. And the whole thing is pretty asexual. If you feel ashamed or self conscious or indecent topless on the beach, or even in a same sex changing room, it's probably not a stretch to think you may feel shame breastfeeding on the side of the road in a car. Now you will have prudes and exhibitionists in any society but the window of normality is going to be a function of what the environment dictates. And iirc being topless on a beach in the US is illegal, whereas no one will blink in Spain or France at a topless woman (except the American/Brit on holiday) But this is learned behaviour - most children don't care about being dressed or not