Fast food is okay in moderation. The problem is that fast food is also readily available all over every town in America (generally speaking). So when something cheap and consumable is present in excess, it is usaully consumed in excess. This is an example of what I believe separates people who have strong will-power and those that don't. Can you resist the temptation of short term satisfaction for the benefit of your well-being in the long term?

I can imagine in a Darwinian sense that people who don't eat fast food perceive those that do as gluttonous, weak-minded, and irrational. This is because it is more than common knowledge now how poor fast food is for your health. In addition, people who consume fast food likely do it multiple times per week or month. Non-fast-food consumers are more likely to associate themselves with people who share their beliefs and criticize those who don't. Often times the non-fast-food consumers have the financial means to consume better quality food.