Twenty-eight states have amended their constitutions to make gay marriage illegal, most of them explicitly defining marriage as between one man and one woman, according to Lambda Legal, an advocacy group for gay rights.
A burst of such constitutional amendments came in the late 1990s and early 2000s, as a backlash to a ruling by Hawaii’s Supreme Court that recognized “different-sex-only” marriages. Some of these amendments also specify that the state is not obligated to recognize same-sex unions licensed in other states.
For example, Mississippi’s constitutional amendment reads: “Marriage may take place and may be valid under the laws of this state only between a man and a woman. A marriage in another state or foreign jurisdiction between persons of the same gender, regardless of when the marriage took place, may not be recognized in this state and is void and unenforceable under the laws of this state.”
States with asterisks have a more complicated policy. Nevada and Oregon have constitutional amendments that define marriage as between a man and a woman, but they also recognize same-sex couples under the separate institution of civil unions. In California, voters in 2008 approved a constitutional amendment endorsing a traditional definition of marriage, but that ballot initiative is now being challenged in federal court.