That's why McDonald's has the Filet-o-Fish sandwich: back when the menu was limited to burgers, fries, shakes, soft drinks and coffee, sales in heavily Catholic communities were dropping off drastically on Fridays.
A bit unrelated, but this reminds me of the time I went to McDonald's in Tel Aviv. I ordered a cheeseburger. The girl that took my order asked back:
"With cheese, sir?"
I was totally surprised by that question. I should have remembered that you don't order meat and cheese together, but still it was on their menu. But why on earth would I order a cheeseburger without the cheese?!? I would have understood better if she had asked "without meat?". Funny people.