Humans are mammals. Mammals produce milk with which to feed their young. Babies are babies. Babies get hungry. Babies don't realize or care that they are on the bus or in the bookstore and not in Breastmilk Bistro or on their couch at home.
Sex is fun.
Lots of people like it. Some people don't. Some people think about sex when they see breasts. You might notice me doing so. Maybe, for some reason, it's not something you want to see.