Nebraska's same-sex marriage ban was struck down by a federal judge on Monday, with no stay placed on the decision.
The ruling by US District Judge Joseph Bataillon will allow gay couples to marry beginning March 9.
Seven same-sex couples had filed suit against the state in November for what they called the "unabashedly gender-specific infringement of the equal rights of its citizens" in court documents.
One of the plaintiffs, Sally Waters, has stage four, metastasised breast cancer, and wants her 2008 marriage to her wife, Susan, recognised partly to provide for their children.
ACLU Nebraska Executive Director Danielle Conrad, who represented the plaintiffs, celebrated the decision.
"Today is a day for celebration," she said. "The love and commitment our clients share will finally be entitled to equality and respect in the eyes of the law."
The voter-approved ban was enacted in 2000, and outlawed marriage, civil unions, domestic partnerships, and other legal relationships between same-sex couples. Governor Pete Ricketts said that the decision should have remained with the state's voters.
"The definition of marriage is an issue for the people of Nebraska, and an activist judge should not substitute his personal political preferences for the will of the people," he said in a statement.
State officials also asked Judge Bataillon to stay the decision pending the US Supreme Court's ruling on gay marriage in the coming months, but he declined.
The gay marriage ban lawsuits in Michigan, Ohio, Tennessee, and Kentucky have reached the High Court, and briefs were filed last week. A decision is expected by the end of June.
"It is time for the US Supreme Court to bring the country to national resolution and end marriage discrimination for all Americans," Freedom to Marry president Evan Wolfson said in a statement.
