[Official Guidance]

DOL Technical Release No. 2013-04: ERISA Definition of Marriage Includes Same-Sex Persons Validly Married in State Where Marriage Was Celebrated
"In general, where the Secretary of Labor has authority to issue regulations, rulings, opinions, and exemptions in title I of ERISA and the Internal Revenue Code, as well as in the Department's regulations at chapter XXV of Title 29 of the Code of Federal Regulations, the term 'spouse' will be read to refer to any individuals who are lawfully married under any state law, including individuals married to a person of the same sex who were legally married in a state that recognizes such marriages, but who are domiciled in a state that does not recognize such marriages. Similarly, the term 'marriage' will be read to include a same-sex marriage that is legally recognized as a marriage under any state law.... The terms 'spouse' and 'marriage,' however, do not include individuals in a formal relationship recognized by a state that is not denominated a marriage under state law, such as a domestic partnership or a civil union, regardless of whether the individuals who are in these relationships have the same rights and responsibilities as those individuals who are married under state law." (U.S. Department of Labor)