While the debate over government recognition of gay marriage is a political hot-button with arguments about morality, civil rights and tradition, the tax issue is a mostly practical one for hundreds of thousands of same-sex couples.
Most states ban gay marriage and don't recognize same-sex unions in any way. Only in Massachusetts can gay couples legally marry. Since 1997, nine other states and Washington started offering civil unions or domestic partnerships that give some or all the legal protections of marriage.
Those protections include allowing gay couples to file state taxes jointly - and potentially save them money. But they can also make tax filing more complicated for the couples.
That's because the state protections do not help with federal taxes. Under the 1996 Defense of Marriage Act, the government defines marriage as being allowed only between a man and a woman.
"You're running one household," said John Traier, a partner in the Butler, N.J., accounting firm Hammond & Traier. "But the federal government and a lot of states treat them as two households."
The same is true for unmarried straight couples who are living together.
There are two main effects of the different treatment under federal law.
One is the tax rate. Take two couples in which one partner has a taxable income of $20,000 and the other makes $40,000. If they can file their federal taxes jointly, the tax bill would be $8,217.50. Filing separately, the combined bill would be $9,032.50 - more than $800 higher.
Another disparity comes with the federal government's treatment of employer-provided health insurance, which also affects unmarried heterosexual couples.
