You are missing one important point. You may be able to keep off the fat, and even lose some of it by doing extreme amounts of cardio or other forms of energy expendature. However the damage to your body by all of that processed junk food isn't lessened all that much. You may be burning it off but you're still setting yourself up for a possible diabetic future, high blood pressure, clogged arteries, etc... Staying active helps but it doesn't solve everything. Unless you are one of those lucky breed of people that can eat anything and never seem to have cholestoral problems, or high blood sugar, fat gain, etc.. then this will catch up to you if you continue on this route. And because you've said in the past that you put on weight easily and fight to keep it off I'd have to guess you aren't one of these lucky few.
I love bad foods as well and eat them in moderation, but eating them all the time and just using exercise to stay in shape is a bad path. Don't confuse being healthy and being in shape as they can be entirely different. Once can look average but have an extremely healthy heart, low cholesterol, excellent blood glucose readings, etc.. Or one can be 6% bodyfat at 250 pounds, but be a walking time bomb. Our goal should be both but many of us on this board worry so much about the latter that we forget the former.