Legally married same-sex couples are now considered married for federal tax purposes. Tax bills could drop for couples if one spouse earns a lot more than the other. But dual-income couples who earn about the same amount will likely end up paying a marriage penalty, just like any married couple. If both partners earn $100,000, filing jointly results in a penalty of $879, according to the Congressional Research Service. But if one partner earns $50,000 and the other $150,000, filing jointly shaves their bill by $557.
If you suspect you’re headed for a penalty, look for ways to cut your tax bill, such as boosting charitable contributions before year-end
Anyone legally married in 2010, 2011 or 2012 should run the numbers to see whether amending two single-filer tax returns to a joint return will save you money. (You aren't required to amend previously filed returns.) You have until April 15, 2014, to amend a 2010 return.