The promised explanation
The disagreement stemmed from the wording on the placard: "There is only our natural world. Religion is but myth and superstition that hardens hearts and enslaves minds." I said that was a general statement about religion, not targeting any group in particular. He said it was offensive to anyone who believed in a religion.
I finally narrowed it down to his view that when someone makes a generalized, strong, negative statement about something, he feels that implies a criticism of the people who hold a different view. Thus, saying "religion is bad" means you are stating that anyone who believes in religion, any religion, is stupid or evil. I handed him the statement from the poll: "Cars are dangerous and destroy the environment", and he replied, "that to me would imply I was either dumb, uncaring, or evil to own a car."
I thought he was being too sensitive, and said as much. I was curious, however, whether I was being insufficiently so, and put up the poll. It seems most of you feel he was being too sensitive about reading between the lines, at least on the metaphorical statement. That's good to know.