On May 7, 1996, the marriage statutes in Alaska were amended to explicitly ban same-sex marriage, although it was never legal before then. That amendment was struck down as unconstitutional on October 12, 2014 and same-sex couples were issued marriage licenses starting October 13, 2014.
On June 6, 2006, the state constitution of Alabama was amended to ban same-sex marriage. That ban has since been ruled unconstitutional.
On November 4, 2008, the state constitution of Arizona was amended to ban same-sex marriage. That amendment has since been struck down as unconstitutional.
On November 7, 2006, the state constitution of Colorado was amended to ban same-sex marriage. That amendment has since been ruled unconstitutional by state and federal courts.
On November 2, 2004, the state constitution of Oklahoma was amended to ban same-sex marriage. That amendment has since been struck down by the federal courts as unconstitutional.
It was explicitly banned there in 1991, although it was not legal before then.
It was explicitly banned there in 1992, although it was not legal before then.
It was explicitly banned there in 1994, although it was not legal before then.
It was explicitly banned there in 1999, although it was not legal before then.
It was explicitly banned there in 2003, although it was not legal before then.
It was explicitly banned there in 2005, although it was not legal before then.
It was explicitly banned there in 2005, although it was not legal before then.
It was explicitly banned there in 2005, although it was not legal before then.
It was explicitly banned there in 2006, although it was not legal before then.
It was explicitly banned there in 2007, although it was not legal before then.
It was explicitly banned there in 2008, although it was not legal before then.
It was explicitly banned there in 2009, although it was not legal before then.