When you marry, you have the option of filing your tax return jointly, or filing separate tax returns. The marriage penalty takes effect when the taxes you pay jointly exceed what you would have paid if each of you had remained single and filed as single filers.
The marriage penalty is the opposite of what many call the marriage bonus. In a marriage bonus situation, you pay less in taxes as a result of your married status. The marriage bonus is most likely seen in partnerships where one spouse earns significantly less than the other. Situations in which one spouse stays at home or has a part-time job rather than a full-time job are most likely to result in a marriage bonus.