Baking a cake for a wedding is celebrating that ceremony and the acts which are the expression thereof. 

Do we want liberal Sharia where Christian bakers are bullied for taking what the Bible says about homosexual activity sinful?  Why is it so important to get a wedding cake from someone with a religious objection when there are plenty of bakers with no objection happy to take the money?  The comparison with segregation is just to invoke an emotional response. There is no Scripture that teaches black and white people can't eat together, but there is Scripture that teaches marriage is between a man and a woman and homosexual sex is sinful.