Well, I still believe that certain fast foods are harmful for your health even when they are taken in moderation. Moreover, taking too much fruits would be much less harmful than taking too much fast food.
Eating too much fruit can be harmful. Only 2 of your 5 a day should be fruit, the sugar content is massive. The main reason is that out taste for sweet things has lead us to breeding fruit with higher and higher sugar contents, all but abandoning cultivation of those strains which are not ram packed with sugar. While the sugars are natural and less likely to cause conditions such as diabetes than processed sugars (although are still a risk factor), they still contain a crap-ton of calories and will make you fat if you eat too much just like anything else. You also end up consuming lots of fibre which can cause digestive problems (namely diarrhea) unless you've been placed on a high fibre diet for a specific reason. Certain vegetarians who consume nothing but fruit are at much higher risk of suffering from conditions such as osteoporosis, again due to the high sugar content of their diets.
I return to my original point. A burger and chips is a burger and chips is a burger and chips (unless it's like one of those Epic Meal Time burgers made with pizzas instead of buns, that's just silly). Coming from a fast food joint does not somehow make it bad, and it can happily exist as part of a balanced diet as long as you're responsible enough to maintain that balance.