No slippery slope, is in Ammendment 14 the equal protection of the laws. If a baker cannot decide who to do business with, the bank can not decide who to do business with. Before this there were laws that applied to blacks that didn't apply to whites and you are claiming we need to go back to it because of "slippery slope".
A baker cannot refuse to do business with a person because of that person's protected status, like gender, race, etc etc.
A baker _can_ refuse to do business with a person that he dislikes, for example with a supporter of a football club that the baker dislikes, or with a person that has caused trouble in the store before.
Banks can refuse to do business for the same reasons. Now the bank manager will not refuse service to a person that he dislikes, not because it would be illegal (it isn't) but because his bank doesn't like losing business. But he will refuse service to a person or company that will cause the bank trouble.