A rather ironic statement considering the condition of my neighborhood. Ok, not so much ironic, but more JUST PLAIN WRONG
For me, going outside into the "fresh air" is anything but healthy. The first thing I notice when I step outside is a distinct tightening of my chest. When I breathe in and taste the pollution-flavoured air, I remember why.
Some days I'm lucky enough to be treated to a variation in the air and I get to play "guess that pullution-tinged flavour!" while I'm walking to class. This game is the spice of my day! Hmmm....what will it be today? Fertilizer? McDonalds hamburger fumes? Or just plain old car exhaust?
Although going out and getting fresh air is very exciting, it does nothing for my lungs or my physical well-being, and, needless to say, will not change my sexual orientation (unless the toxic soup of chemicals in our air has acquired some new powers since I stopped reading the newspapers).
So when your father says this to you, feel free to laugh at him. I know this will probably not make you feel any better or improve your situation, but hopefully he will understand the complete innacuracy of his statement. You can tell him that it would, in fact, be healthier for you to remain inside typing away on your computer for 24 hours of every day. After laughing at him, I guess you could reply with "Sure, dad! I'll step outside to brave the carcinogenic whirlpool of dangerous chemicals that will immediately invade my lungs. God knows it would be healthier than staying in here!"
That's what I would do. Well, I'd probably just run away or move or something, but I'd definitely think of this afterwards and wish I hadn't run away. Good luck. :)