Why does our society make nursing your child unacceptable?

I'm a nursing mother and I find it hard to feed my child when out in public. Going out to eat, I have to feed my daughter in the bathroom (gross) or out in the car in order to avoid dirty looks from others. I'm very discreet about nursing so I don't understand how people can see it as 'lewd' or disrespectful. To me, my aren't even objects anymore- they're bottles. How can something so natural be so unacceptable to the public?