Same sex couples are not recognized when filing federal income tax returns. In 1996, the Defense of Marriage Act defined marriage as between one man and one woman, prohibiting federal recognition of same sex married couples and polygamists. So, same sex couples cannot file joint federal returns.
Some states, however, do recognize same sex marriages, domestic partnerships, or civil unions. In these states, a same sex couple may file a joint return for their state taxes. As of now, these states are Massachusetts, California, New Jersey, Connecticut, New Hampshire, Oregon, Vermont, Washington D.C., and the state of Washington.
Massachusetts is the only state recognizing same sex marriage. Therefore, any legally married same sex couple in Massachusetts can file a joint state return.
Since tax year 2007, you can file a joint return in California as a same sex couple.
If you are married or in a civil union by the last day of the tax year, you may file a joint return in New Jersey. If, however, you are not married or a partner in a civil union, you may not file a joint state income tax return. This especially applies to registered domestic partners, which status does not qualify to file a joint income tax return in New Jersey.
Since tax year 2006, civil union couples can file a joint state income tax return in Connecticut.
Since tax year 2008, if you are a civil union couple residing in the state of New Hampshire, you can file a joint state income tax return.
Oregon has a very complicated domestic partner tax structure. In some cases, same sex couples may be able to claim a subtraction status from income, but by no means under all conditions. It is best to check with the state of Oregon department of Revenue for more details.
If you are registered civil union partners in Vermont, you can file a joint state income tax return.
Since 2006, registered domestic partners in Washington, D.C. can file a joint income tax return.
The state of Washington does not allow joint filing for domestic partners.
These state regulations are often in flux due to the constant battle and litigation over same sex couples rights. At almost any point in time, it seems the laws governing civil unions and domestic partnerships can be changed or modified. It is highly recommended that you check with the Department of Revenue in your state in order to be up to date regarding the income tax filing requirements and restrictions regarding civil unions and domestic partnerships.