I had a similar issue except we have 3 cars, but only a 2 car garage. My wife's car, a SUV, gets a spot in the garage as well as my Boxster. My DD sedan sits outside. When we started collecting more and more junk (snow blower, generator, etc) that started taking up more space, wife suggested I move the Boxster out to fit all this junk. Told her no way, and that instead, I'd move said junk into a shed. If her car can stay in the garage, why can't mine? You gotta put your foot down.
