Except that it's a myth that fast food is especially bad for you.
http://skeptoid.com/episodes/4088
It's no myth. The story in the link is absurd.
For example they point out eating three double quarter pounders with cheese a day would be a good, healthy, slim 2,200 calorie diet for an adult. However, they don't mention that would be 126 grams of fat! Not to mention it would be severly deficient in vitamins and other nutrients. And would contain enough salt seriously put you at risk of hypertension, for many people, if they ate them every day.
The article basically uses the "cigarette" defense. Since you can't prove that eating a high fat, high sodium, low roughage diet is the direct cause of a health problem, then its not.
Basically fast food is high in calories, fat and salt. Not to mention full of artificial flavorings and preservatives, which MAY cause health problems. Especially if eaten in excess.
All of which would be fine if it were an occasional meal.
Eating fast food regulary would make it very difficult if no impossible to eat anything like a recommended healthy diet.