And if you have 2 people living in a house each making $10,000 a year, what about $2,000 a year versus $20,000 a year? More palatable, right?
That's the problem with making exceptions. Let's say there's a flat tax of 10%, and a guy is making $100,000. Due to the graduated flat tax, if an individual is making less than, oh, say, $60,000, they only pay 8% versus 10%. So what does the guy do? He apportions half of the income - or possibly, 40% - to his spouse. They both pay at the 8% rate instead of the 10% rate, and voila, they've ducked $2,000 in taxes simply by creative accounting.
For those that espouse exceptions to promote fairness, you must acknowledge that those exceptions are also opportunities for others to exploit, and end up being even less fair in the long run.