Tell them its not about pedaling all the time, its about the overall feel of the fixed ride. Let them try you fixed gear bike (in a controled envioronment like a parking lot or park) so they can see for themselves. Almost anyone can ride a bike with brakes and freewheel, riding a fixed gear well is special skill and like all things that require sill and practice, it comes with a reward.

Fixed gear to me (after many years or riding road and mountain bike) bring me back to the simple days of riding BMX as kid. Simple machines often provide the purest and most rewarding rides.