I have a wife, and a baby daughter. I tell my wife that my biking is about three things:

1) Transportation (I commute by bike)
2) Hobby

and the big one:

3) Fitness

Pull the "I want ot be around to see our grandchildren, and be healthy enough to enjoy it" card.

I regularly pad my commute home with an extra half hour of riding, and occasional weekend rides.

Now I'm not a competitive rider or anything, that would be a whole different ball of wax. But everyone needs hobbies.