Well actually, I could argue that obese people do hurt me because hospital resources and tax money is spent treating them and that the increased burden on the NHS does hurt me if I ever need treatment.
I think McDonalds is just a symbol of obesity really. There are many, many ways in which obesity has become a problem and McDonalds is simply one of them. I don't think it's the central cause at all, but it and other fast food chains are certainly a contributor.
As for the food itself, it's the type of restaurant I'll only visit if nothing else is on offer. Like if I go to Liverpool on the train and I've only twenty minutes or so in Chester to get something to eat, McDonalds is an option then because it's quick to get and run back to the train station with. If something other than fast food is on other, such as sandwiches (not Subway because I find it too expensive), I'll take that instead. Besides, a portion of chips from the chippy is cheaper (and nicer) than a meal from McDonalds if I want something unhealthy.