I agree with W8in4dave. People just don't know. It doesn't make it okay that they react like ***hats, but normally when people act like that, I choose to feel sorry for them. I'm sorry that they have so much anger inside that MY diet is somehow offensive to them. I'm sorry that they are so reactionary that they let something small like that get them in a tizzy. It has really been occurring to me a lot lately that negativity gets us nowhere (Which is not to say that the OP was being negative! Not at all.) Just that hanging onto it and making it a part of me, rather than just choosing to let it go, isn't GOOD for me. I've actually recently stopped following a few of the gluten-free blogs that I was really into for a while because I feel that they are just too negative, with too much focus on anger. They aren't uplifting.
So for the OP, as cheesy as it sounds, when people act like jerks to you, I think it's good to practice just letting it go. Send them love and light and good vibes. They're probably hurting inside in some way, ya know? =-) In the end, the only thing that we can control is how WE react to a situation. And like Gemini said, a big, kind smile always throws them for a loop.