I personnally as a rule don't swear in day to day life, stubbing toe, numpty pulling out in front off me in car etc aside and only if kids are not around.
But When at the football that is a place where I let rip. Its supposed to be a passionate game and passions can run high at times. When I have taken my son(aged 12) to a game I tell him that I might say the odd dodgey word and its part of supporting your team especially when people like Brewster are ruining the team you support. This hasn't turned him into a fowl mouthed chav. He knows its part of the game and has said that school its as bad if not worse.
I have a bigger gripe with the mothers who stand outside my youngest boys (aged 5) classroom at beginning and end of the day,swearing, smoking, talking about what they did to their latest boyfriend at the weekend. That I think is more inappropriate the swearing at a game of football