When it comes to sectarian bigotry, Scotland is a backward country. You can try and take context out of the discussion, but anybody with an IQ bigger than their shoe size knows that taking a Union Jack to a Celtic game is different from taking one to Last Night of the Proms.
All this "but it's just a show of Britishness" is at best, disingenuous.
I'm all for freedom of speech and 100% believe you should be allowed to display whatever flag you like. However, freedom of speech also gives me the right to say it drags us down to the OF level; something I don't want to be associated with my club.