for federal income tax purposes, the term spouse includes an individual married to a person of the same sex if the couple is lawfully married reg 301.7701-18. however, individuals who have entered into a registered domestic partnership,civil union, or other similar relationship that is not considered a marriage are not considered married for federal tax purposes.
(b) Persons who are lawfully married for federal tax purposes -
(1) In general. Except as provided in paragraph (b)(2) of this section regarding marriages entered into under the laws of a foreign jurisdiction, a marriage of two individuals is recognized for federal tax purposes if the marriage is recognized by the state, possession, or territory of the United States in which the marriage is entered into, regardless of domicile.
(2) Foreign marriages. Two individuals who enter into a relationship denominated as marriage under the laws of a foreign jurisdiction are recognized as married for federal tax purposes if the relationship would be recognized as marriage under the laws of at least one state, possession, or territory of the United States, regardless of domicile.
(c) Persons who are not lawfully married for federal tax purposes. The terms spouse, husband, and wife do not include individuals who have entered into a registered domestic partnership, civil union, or other similar formal relationship not denominated as a marriage under the law of the state, possession, or territory of the United States where such relationship was entered into, regardless of domicile. The term husband and wife does not include couples who have entered into such a formal relationship, and the term marriage does not include such formal relationships.
these states allow additional filing statuses for same-sex couples:
California
Connecticut
District of Columbia
New Jersey
Oregon
Vermont
