Outside or Inside Dog?

My wife and I have different perspectives on where our 4-year-old Boxer shouldspend his days. My view is that he shoudl be outside (fenced, 1/4 acre backyard). She prefers to let him out as needed but not to leave him out for extended periods. While he is still lean and muscular, I am concerned that his indoor lifestyle may lead to weight gain. Also, I just think he is happier being lopping around our back than lying on the living room floor. I think my wife is concerned that he would become a "dirty outside dog" - the kind that you find on farms - if we let him stay outside 8 hours a day. Anyone have any ideas?