What other religious reason would cause someone to refuse to fulfill their business obligations based on something arbitrary like the sex of the customer?
Homosexuality is a repugnant practice according to the teachings of all three of the world's largest monotheistic religions.
Mullins and Craig were planning their wedding in Massachusetts in 2012 and wanted the cake for a reception in Colorado, where gay marriage was not yet legal. During a brief encounter at Phillips' Masterpiece Cakeshop in the Denver suburb of Lakewood, the baker politely but firmly refused, leaving the couple distraught.
Phillips' lawyers said his cakes are an art form - a "temporary sculpture" - and being forced to create one to commemorate a gay wedding would violate his constitutional rights to free speech and expression and free exercise of religion. Phillips and others like him who believe that gay marriage is not consistent with their Christian beliefs have said they should not be required to effectively endorse the practice.
They filed a successful complaint with the state commission, the first step in the six-year-old legal battle. State courts sided with the couple, prompting Phillips to appeal to the top U.S. court.
The case's outcome hinged on the actions of the Colorado commission. In one exchange at a 2014 hearing cited by Kennedy, former commissioner Diann Rice said that "freedom of religion, and religion, has been used to justify all kinds of discrimination throughout history, whether it be slavery, whether it be the Holocaust."
Justice Kennedy noted that the commission had ruled the opposite way in three cases brought against bakers in which the business owners refused to bake cakes containing messages that demeaned gay people or same-sex marriage.
https://www.cnbc.com/2018/06/04/supreme-court-rules-in-favor-of-colorado-baker-who-refused-to-make-wedding-cake-for-gay-couple-for-religious-reasons.html