1 U.S.C. § 7 : US Code - Section 7: Definition of "marriage" and "spouse"

      In determining the meaning of any Act of Congress, or of any
    ruling, regulation, or interpretation of the various administrative
    bureaus and agencies of the United States, the word "marriage"
    means only a legal union between one man and one woman as husband
    and wife, and the word "spouse" refers only to a person of the
    opposite sex who is a husband or a wife.