It does have an effect on many Christian businesses already: did you read about the baker who refused to bake a wedding cake for a gay couple's wedding? He is being sued!
IF it ever becomes legal throughout the U.S., everything/anything that is refused to married gays will be grounds for a lawsuit. Will churches be required--mandated--to perform same-sex weddings? They have already had to close the doors of Catholic Adoption agencies for their refusal to allow gays to adopt because the agencies "served the general public," so were disallowed to retain their Christian beliefs. But then again, perhaps the Left especially WANTS to see more churches and Christian business either kowtow--or close.