Judge C. Scott Crabtree pulled no punches in his 49-page ruling, saying the state's voter-approved ban "bears no rational relationship to any conceivable government interest."
The ruling makes Colorado the latest in a string of 16 states that have seen their bans on same-sex marriages tossed out by state and federal judges.
The ruling came as another judge in Boulder County considered a request by Attorney General John Suthers to stop a county clerk from issuing marriage licenses to gay couples. And last week, attorneys filed a federal lawsuit seeking to challenge Colorado's gay marriage ban.
Attorneys for 18 plaintiffs — nine couples — from Denver and Adams counties argued that the state's ban on same-sex marriage violates the U.S. Constitution.
"We are ecstatic. There is much cheering in our house," Sandra Abbott said after Crabtree's ruling. She and her partner, Amy Smart, were one of the nine couples in the lawsuit. "We waited a long time for this ruling."
Suthers' office said it will appeal the ruling.
