Originally Posted by Steve M

I understand people that would prefer to avoid McDonald's when overseas, out of the notion that they want to try local things. And I understand people that avoid any fast food like the plague under any circumstances. What I don't understand is those that wouldn't ordinarily be adverse to eating at McDonald's when home (even if it's not their first preference) but have this absolute rule against it when traveling in another country.

I get part of it: you want to try local things. I do too. In most cases, I'd prefer the local option in most cases.
I once heard Julia Child give an interview where she was asked if she ever ate at McDonald's, almost as a joke question. She said that Yes, in fact she did. She traveled a lot, and sometimes it was the best option: she knew exactly what she liked and didn't like on the menu, and exactly how it was going to be prepared, and sometimes didn't want to experiment with the unknown when she had a place to be at a set time and no other opportunity to eat. If she can do it, I'm sure some of the sophisticated gourmet world travelers could lower themselves on occasion.