“This begs the question, if Mr. & Mrs. Knapp were Black ministers and they refused to marry klan members, White supremacists, etc., or if they were rabbis who refused to marry neo nazis or Holocaust deniers, would they still face prosecution? “
even though I like the idea of your question, I think their argument would be the groups you mention are not protected groups, notwithstanding the fact that in a country whose basic principles are all men a created equal, we have , by law, created groups that are unequal.
First Amendment issue. Citizens arrest of anybody trying to arrest the ministers.