On September 21, 1996, H.R.3396 - Defense of Marriage Act was signed into law by President Bill Clinton.
The law states:
Defense of Marriage Act - Amends the Federal judicial code to provide that no State, territory, or possession of the United States or Indian tribe shall be required to give effect to any marriage between persons of the same sex under the laws of any other such jurisdiction or to any right or claim arising from such relationship.
Establishes a Federal definition of: (1) "marriage" as only a legal union between one man and one woman as husband and wife; and (2) "spouse" as only a person of the opposite sex who is a husband or wife.
On June 26, 2013, in the case of United States v. Windsor, by a vote of 5-4 the U.S. Supreme Court ruled that the Defense of Marriage Act is unconstitutional.
Read the 77-page "Opinion of the Court" here.
Between the Lines of the Defense of Marriage Act Opinion | The New York Times
Same-Sex | Gay Marriage in the United States | Health and Wellness Resource