Really? LOL! Sounds like yobs to me! Ask them what they consider golf or car racing or fishing to be... if they reply "sport", then you know they're morons.
I guess I don't have a problem with it, since I know that is SUCH a stupid statement to say. Quiz them and find out what defines a "sport" for them.
I'd be curious to hear their replies.
Tell them this as well:
You compete against others in MTB (up to olympic levels)
MTB requires fitness and strength and dexterity
You can get seriously hurt in MTB
You can operate in teams
You have to train if you want to compete
I guess the biggest differences between footy and MTB is that you can do MTB alone without looking like an idiot, you don't need to compete against anyone to have fun, there is no "ball".
Heck, that is just so dumb what they said, I wouldn't even bother asking them.
See... this is the problem we have when we let cousins marry...