fast food is a by-product of the modern world.... just like emissions from cars, the harsh chemicals we breathe and expose to our skin, the pharmaceuticals we ingest to combat ailments: these are all things that any modern western civilization has now embedded in its nature, however un-natural they may seem....
i see things like fast food like an immunization for some of the world's ailments.... you don't need very much of it, but eating foods that are laden with chemicals, fat and salt every so often can train the body to metabolize it efficiently and without any long-term effects.... but just like anything, if you eat too much fast food, you're likely to succumb to the ailments that long term exposure and consumption of these products can bring to the body.... it's like over-consuming a drug, drinking too much coffee, or even eating too many carrots... as long as everything is consumed in moderation - and that's a relative thing - then people shouldn't have any problems....
and to all the city dwellers who live with air purifiers, water filtration systems, and a 100% organic diet: MOVE TO THE FREAKIN' COUNTRY! your bodies are not equipped to handle the city and it will ultimately make you very sick....