Its funny because there is a similar thread going on right now on the Adoptive and Foster Parenting board about others trying to justify (as if they have to!) why THEY "cant adopt" when they see a family with adopted children. For some reason some people feel like another person's decisions is somehow a comment on THEIR own decisions (like, if my kids are adopted, someone im thinking you should to? but really im not!)...no matter if its formula/breast, home/daycare i think people want affirmation of their own decisions or something. i dunno.
Often too, i think people are just surprised by the "unusual" and say stupid stuff in the moment without meaning to be offensive. Or they dont think about how it may be funny to THEM but its the 100th time you've heard that comment. My oldest has red hair and every.single.time we left the house he heard "where'd ya get that red hair??" or i'd get "does his father have red hair??" EVERY TIME WE LEFT THE HOUSE. people just dont think. I usually tried to think of them as being curious or making smalltalk rather than rude though.