I know - people definitely say dumb things sometimes. It makes me really wonder what they are thinking!
I think it depends on how it's said, also. One time a lady from the Phillipines stopped me and said, "He does not look like you! You are an American! Is he Filipino?" She said it in such a bewildered state I just realized maybe the concept of adoption wasn't really familiar to her.
As a side note, one time after Hector came home (it had been just a few days), I found myself referring to his "real mother" back in Guatemala. I guess it just hadn't sunk in yet! So now, I really try to think before I speak!