But why? One meal once a week is less than 5% of your food intake, surely it's your overall diet that matters and the odd fast food meal once a week isn't going to undo an overall healthy diet.
There's a man in the US who has eaten nothing but Big Macs (three times per day) for decades. He's not overweight and looks healthy. He probably has a better diet than someone who lives on cereal, toast and instant noodles.
I travel around for work and while McDonalds would never be my first choice, they're everywhere, cheapi for a hot meal, quick and you know what you're going to get.
Most of the time I don't have time to walk into a random cafe only to find it's one of those places that takes 20 minutes to produce any food. I don't like burgers but don't mind a fish fillet and fries, chicken wrap and fries or sausage bagel if it's breakfast time.
Sometimes I use supermarket cafes as well and last week I had scampi and chips in Morrisons and while the portion size was quite small (10 pieces of scampi, I counted, a handful of chips, most of which were undercooked, but I didn't have time to complain and wait for more to be cooked and a couple of tablespoons of mushy peas, the menu said it was nearly 1100 calories and I was still hungry afterwards.
If had my usual McDonalds order of fish filet, medium fries and a coke zero, that would have been around 650 calories and I would have been stuffed.