None of that is unlawful discrimination, because it is not arbitrary discrimination. Unlawful discrimination occurs when an advertised service is withheld because of who the customer is, not because of the behavior the customer exhibits or the demands the customer makes.
As I said above, the solution may well be quite simple - just have a wedding-related business state that it caters to religious marriages only. Civil marriage and religious marriage are not the same thing, and no church should be compelled to convey any spiritual significance to a civil marriage, or to solemnize same. Taking that concept into the realm of commerce, I think that all a baker or calligrapher or photographer needs to do is properly advise the public that its services are limited to the celebration of religious marriage.
ar·bi·trar·y
ˈärbəˌtrerē/
adjective
based on random choice or personal whim, rather than any reason or system.
"his mealtimes were entirely arbitrary"
synonyms: capricious, whimsical, random, chance, unpredictable; More
(of power or a ruling body) unrestrained and autocratic in the use of authority.
"arbitrary rule by King and bishops has been made impossible"
synonyms: autocratic, dictatorial, autarchic, undemocratic, despotic, tyrannical, authoritarian, high-handed; More
MATHEMATICS
(of a constant or other quantity) of unspecified value.
I don't think it means what you think it means.
Do you realize how silly it sounds? These people are suing because they were informed that the business limits its services. Why do we need a written sign to counter an unspoken misconception on the part of the would be customer. Informing them verbally should be sufficient. If not, why not?