Actually, there is. Of one believes as the Catholic Church teaches that "gay marriage" is not something that can morally be supported by a practicing Catholic, then baking a cake for said "marriage" would be sinful. At that point, a business person decides whether it is more important to serve God or Caesar. If you require Biblical citation, I suggest you begin by reading St. Paul's Epistles concerning the morality of a good Christian. Try 1 Corinthians 6:9. There are others.