Frankly I'd just ignore them. Go silent and walk away. Eventually they might get the picture. Surgery of any kind is no one's business but your own. They don't have any rights, though they may believe differently, to comment on nor ask why.
I have gotten the same comment about not needing surgery because I was small chested...from a therapist no less! I was very upset with her and dropped her faster than a hot potato once I got my T letter....which it turns out I did not need. And quite honestly unless these people have seen you minus clothes and binder they can't form an honest opinion about such things.
My point in telling you this is that no one has any right to make comments like that about a surgery that you, yourself have deemed necessary for your own peace of mind and well being.