Ah I love love love this topic, and I'm especially interested in class issues in food.
Just a few parameters/thoughts:
1. We're talking about a few categories of food here.
"Fast food" (the chain, multinational store kind), "fine dining" (the independent, sit down, expensive, small food on big plate kind), and "home food" (the kind you associate with a village, farm, your grandmother, Rachael Ray, etc.).
2. It's kind of funny, because the way I see it, fast food can be both "healthy" and "unhealthy", as with fine dining, as with home-cooked food.
I once had a pretty clean egg white cheddar sandwich from some random shop in Boston. It was as fast as fast food gets, and it was $2. I also once had lovely gnocchi in butter and shaved parmesan in a swanky Italian restaurant in downtown SF. I think I did better healthwise with the $2 sandwich.
You'd probably do better eating a turkey subway than a hearty risotto. But let's say you're a single mom working two jobs. You don't have time to stir and stir with a serene Alice Waters-esque face on, plus you have a gazillion things on your mind, so you grab a fast food $4-ish turkey sub while rushing to your next shift.
3. When you're poor, "healthy food" might be lower down your priority list.
First off, compulsory reading for anyone claiming to be interested in both food and social justice issues. This is Linda Tirado, she talks about poverty and food choices - http://www.theguardian.com/society/2014/sep/21/linda-tirado-poverty-hand-to-mouth-extract
Let's just imagine fast food is indeed all unhealthy junk, way more terrible than any restaurant food.
It's still cheap, it's still fast. Without even talking about taste, cheap + fast still means a lot to an exhausted working class person with a tiny lunch budget. I've never been in this situation, lucky me. But I'd totally understand a person with zero f**ks to give about whether their lunch is gluten free, low fat; too spent on a tight budget to count their calories.
4. Actually, I have more questions about the moral and aesthetic meanings we attach to food.
How did we learn to self-flagellate over food? And how did we develop an eye for presentation? The very moment we talk about "food choices", we enter a mode of thinking that assumes everyone thinks of food as choice, a representation of bodily discipline and a refined palate.