Yeah i know how you feel, it happens to me a lot, but that is those peoples problem, too bad they dont get to find out what a sweetie your poochie is.
But then again i will sometimes get a different reaction, we went to the Bark in the Park a while back when we had our other Rott (Bear) and plus we had Face, and some lady came running up to us with her new born baby to pet them! Wow i was shocked i dont know how many people would want to introduce there new born to 2 strangers big Rottweilers, but it was nice to see not everyone acts like all big doggies are viscious!