Actually, according to the Tax Foundation:

-A marriage penalty or bonus is the change in a couple’s total tax bill as a result of getting married and thus filing their taxes jointly.
-Marriage bonuses typically occur when two individuals with disparate incomes marry.
-Marriage penalties occur when two individuals with equal incomes marry; this is true for both high- and low-income couples.
-Marriage bonuses can be as high as 20 percent of a couple’s income, and marriage penalties can be as high as 12 percent of a couple’s income.
-While research shows that marriage penalties and bonuses do not have much effect on whether a couple will marry, they do impact how much each spouse works.
-It is possible to completely eliminate both marriage penalties and bonuses, but it would require a significant overhaul of the tax code that drastically changes the current distribution of income taxes paid.

So this tax code helps couples which make enough money to survive on one couples income and the incentive for one person to stay at home becomes even more extreme if you have young children (because you have to pay childcare costs in order to work.)

There have been many times in my marriage where I quit working simply because when I calculated childcare costs, the fact that we ate out more when both of us worked, gas, teaching supplies, quality of life, and the fact that when I worked it put us in a much higher tax bracket .

In retrospect, my actions had unintended consequences--when the credit bubble burst, had I still been teaching--we would have had that income to fall back on. Our tax code was definitely a factor in my tendency to change and quits jobs. And most unfortunate of all, lower income couples are just stuck with these rules (cannot game the system by one person staying at home) which were likely passed when fewer women were part of the work force. So much about our tax code is so archaic!