My favorite comebacks:

To: "Are you still BFing???":
"Yes, isn't it wonderful!!!"

To: "How long are you going do THAT?"
"Until he's satisfied!" or "As long as he wants to!"

To: "How can you sit here and do THAT?"
"I'm quite comfortable thank you!" or "Would you rather I stand up?"

To: "You know, they have bathrooms for THAT!!!"
"Would you like to bring your lunch and join me?"

The most frequent comments that I have gotten from strangers:  "That baby
sure does sleep a lot!", "You couldn't tell there was a baby present!",
"Doesn't that baby ever cry?" and "What a happy baby!".  My response:
 "That's because he's breastfed!"

A while back, a nursing mother told me that she was sitting on a bench at the
mall breastfeeding her baby when a middle aged gentleman with an accent
approached her and said, "I've been living in this country for over 20 years
and this is the first time I've seen a baby being fed properly!"  She was
surprised by his comment for more than one reason.

The best 'all purpose' comeback that never fails!!!  Look at the speaker,
square in the eyes pretending that you don't understand a word that they have
said, give them a beautiful 'Thank you type' smile with a little nod, and
then immediately go back to admiring the wonderful gift in your arms.  Only a
fool can't see love!

Leona Simoes, LLLL