That they worship the devil.
DH had a friend who when he learned that DHs wife was pagan flipped out about how we worship the devil. DH explained that the "devil" is a christian construct so how could pagans worship something they don't believe in??
The friend went on about the pentagram and DH pointed out that it was upside down, to which friend said, "What difference does that make?" DH said, "Does an upside down cross mean the same thing to you??"
This has come up many times.