Yesterday's ruling says Utah's law passed by voters in 2004 violates gay and lesbian couples' rights to equal protection.
Attorneys for the state argued that Utah's law promotes the "optimal mode of child-rearing."
The lawsuit was brought by three gay and lesbian couples.
Similar challenges are pending in other states, but Utah's is closely watched because of its history of opposition to gay marriage as home of The Church of Jesus Christ of Latter-day Saints.
It's unclear what the immediate effect would be, because the state can appeal.