I understand that, the point of the question was, if someone decided not to use vague rules, but instead decided to stand on principle and decided proclaim publicly that anyone that would celebrate the holocaust was offensive to them and base on that they would refuse to serve them.
I do not think that people who would like to celebrate the Holocaust are a protected class.
I do not see a reason for me or the state to care about that interaction.
On the other hand, you may be asking us about someone who finds gay marriage and the Holocaust to be morally comparable.
What if the hypothetical Christian bakers' sincerely held beliefs about gay marriage is the same as or equivalent to some hypothetical Jewish bakers' sincerely held beliefs about the Holocaust?
Would it then be acceptable for the hypothetical Christian bakers to refuse service based on their sincerely held beliefs about the heinous and atrocious nature of gay marriage?
Legally the question may be simpler than the moral question as if there is not a law prohibiting that sort of discrimination, then we're good to go.
"Should there be a law which could force someone to act against their sincerely held beliefs?" is a different question.
The answer is, "Sometimes."
Sometimes there is a compelling interest that the government should require all who conduct certain sorts of business to conduct that business in certain sorts of ways.
Those who are unwilling to do so are forced to operate illegally or change their business.
There may have been and may still be some places of public accommodation whose owners chose to close because of the civil rights legislation or who chose to continue discriminating against potential customer based on race despite the legislation.
The law changed after many of them had opened their businesses.
I am not sure about the degree of the necessity of anti-discrimination laws for the LGBT community.
It seems that public opinion is strong enough [except maybe among the older generations] and growing stronger that the issue will largely fade of its own inertia.
I have no real objections to the theory of some anti-discrimination laws either.
imho the sentence, "I would tell a potential customer not to give me his money because..." has very few rational endings