Isn't his what we've been hearing recently from progressives: someone running a small business can't decline to be a part of a religious ceremony?
This has nothing to do with a business picking and choosing who they serve from the general public.
Never mind the fact that gay marriages aren't religious ceremonies.
Are they different than other marriages in this respect? Even if it is not a religious ceremony for some because they have removed the religious belief and are left with only its ritual form and setting, you are telling someone the must perform for something that to them has a religious significance counter to their beliefs. It is like telling a Jehovah's Witness he must say the Pledge of Allegiance, or telling the Quaker he must swear an oath in court.