I say "I'm having a lot more fun, riding a lot farther, and I'm still grinning when I get done. What's cheating about that?"
If they want to take it further, I refer to my Fitbit results.
My "standard" ride is 45 miles with a 1600-ft elevation gain in the middle, which takes me about 3 hours. My Fitbit reports about 50% of the time in fat-burning zone, 30% in cardio zone, and 20% in peak zone. I then ask them how their 3-hour rides go...