Gov. Rick Perry signed a law preventing Texas from legally recognizing same-sex unions that are formed in other states.
Tuesday's signing of the Defense of Marriage Act (search) makes it the 37th state to enact such a law, said Kelly Shackelford, president of the Plano-based Free Market Foundation, a group that describes itself as dedicated to strengthening families.
Already, Texas only permits marriage between a man and a woman.
"What this does, it protects your state from having a different definition forced on you from another state," Shackelford said.
Critics say the bill represents a right-wing agenda of intolerance.
"It's about politics. It's about scapegoating gays and lesbians," said Randall Ellis, executive director of the Lesbian Gay Rights Lobby of Texas (search).
Ellis said the act was unnecessary because existing statutes specify that, in Texas, state laws apply even to marriages performed elsewhere.
Supporters said the act was needed to protect Texas from the decisions of judges in other states where same-sex unions could be recognized.
"Like the vast majority of Texans, I believe that marriage represents a sacred union between a man and a woman," Perry said in a statement. "With passage of the Defense of Marriage Act, Texas now joins more than 30 states in reinforcing that basic belief."