Well, in case you haven't gathered it yet, no, fast food isn't entirely, "bad" for you. As far as any diet should be structured (in my opinion), you should have a daily set macronutrient (carbs, fats and proteins) intake, and you should strive to hit that goal. This has personally worked well for me and many others that I know. I understand not everyone has the drive, need, or ability to do this. If you would rather not, I would say go ahead and blow the heck up on that fast food. You could easily add thousands of more calories by hitting McDonalds or Taco Bell. For a lot of people (as mentioned above), this hinders their training. I know personally, I just don't feel as good all around when I eat mass amounts of McChickens and McDoubles as opposed to not intaking McDonalds. In general, if calories in > calories out, you will gain weight. Just how it works. I know it has been a while since you have posted so I hope all is going well with you and your mass gaining! Good luck to you and I hope I at least somewhat helped!