The order from administrative law judge Robert N. Spencer said Masterpiece Cakeshop in suburban Denver discriminated against a couple "because of their sexual orientation by refusing to sell them a wedding cake for their same-sex marriage."
The order says the cake-maker must "cease and desist from discriminating" against gay couples. Although the judge did not impose fines in this case, the business will face penalties if it continues to turn away gay couples who want to buy cakes.
The American Civil Liberties Union filed a complaint against shop owner Jack Phillips with the Colorado Civil Rights Commission last year on behalf of Charlie Craig, 33, and David Mullins, 29. The couple was married in Massachusetts and wanted a wedding cake to celebrate in Colorado.
The commission is expected to certify the judge's order next week.
Nicolle Martin, an attorney for Masterpiece Cakeshop, did not immediately return a call seeking comment. Phillips had argued that making cakes for gay wedding ceremonies violates his Christian beliefs.
Mullins and Craig wanted to buy a cake in July 2012, but when Phillips found out the cake was to celebrate a gay wedding, he turned the couple of away, according to the complaint.
A similar case is pending in Washington state, where a florist is accused of refusing service for a same-sex wedding. In New Mexico, the state Supreme Court ruled in August that an Albuquerque business was wrong to decline to photograph a same-sex couple's commitment ceremony.
Colorado has a constitutional ban against gay marriage but allows civil unions. The civil union law, which passed earlier this year, does not provide religious protections for businesses.