Phillips contends the U.S. Constitution's free speech guarantees protect him from making a cake that would violate his religious beliefs against gay marriage. To Mullins and Craig, the baker's refusal represented a simple case of unlawful discrimination based on sexual orientation.
In one of the biggest cases of the conservative-majority court's nine-month term, the justices -- just two years after legalizing gay marriage -- must decide whether Phillips' action was constitutionally protected and he can avoid punishment for violating Colorado anti-discrimination law.
A ruling favoring Phillips could open the door for certain businesses to spurn gay couples by invoking religious beliefs, as some wedding photographers, florists and others already have done.
The brief encounter at Phillips' Masterpiece Cakeshop in the Denver suburb of Lakewood left Mullins and Craig distraught. They filed a successful complaint with the Colorado Civil Rights Commission, the first step in the five-year-old legal battle that the nine justices will resolve in a ruling due by the end of June.