I admit I kind of like some of the snarky replies above. "Why are you so intent on viewing my breasts?" "Here's a picture for you to look at while I go nurse."
I'm with you, OP, I'm not a mom and never have been, but I find it intensely irritating, in general, when people try to make me feel bad for not being as relaxed, liberated, comfortable, free, etc. etc. as them. As in I must be incredibly stifled not to want to go to a nude beach, or a male stripper act, or, (in my teens) dye my hair a weird colour. (After taking some flak from a friend with naturally mousy hair who liked to dye her hair bright orange, or black, or blue, when I didn't want to dye mine, I got her to quit nagging me by saying, "Some of us actually don't hate our natural colour.")
In your shoes, I would probably go with something like, "This is the way I choose to nurse and will continue to choose to nurse. I respect other women's choices, including yours, so please respect mine and stop pestering me about it."