Here’s the relevant passage of the “Defense Of Marriage Act” 5/96 H.R. 3396

(a) IN GENERAL.-CHAPTER 1 OF TITLE 1, UNITED STATES CODE, IS AMENDED BY ADDING AT THE END THE FOLLOWING:

“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.”.