Even with fast food there are reasonably healthy options. McDonald's had grilled chicken sandwiches and wraps. Wendy's has salads. Subway always has something healthy. I'm sure it is unhealthy in the sense that the food was probably treated with growth hormones, etc., but I mean not everything there is high in calories, fat, sodium, etc.
In my own case, I'm not exactly a healthy eating nut, but I do try to minimize unhealthy foods (including fast food, but also unhealthy foods available at "real" restaurants and at home), but I splurge from time to time. The rule of thumb for me is reigning it in, not eating until I'm full, making sure I balance things out by eating healthier if I ate more unhealthy the day before, etc. I don't want to go to extremes either way, because for me, part of enjoying life is eating good, sometimes unhealthy food. But moderation is key. The older I've gotten, I have found that this basic rule works in almost every aspect of life--work, exercise, relaxation, play, relationships, etc.