The issues as I see them:

Restaurant is a public accommodation, but turning away bigots because they offend your staff is not discriminating based on religion. They, of course, want to frame it that way. But I would bet 90% or so of the guests were Christians so its a little weak.

Creatively, they could argue that it’s discrimination based in their particular *brand* of religion, and that may be more interesting. Restaurant seems to have a defense though, and right wing anti-gay bigots are not a protected class last I checked. I do think they could advance that argument in front of the right judge though if they had good lawyers who know how to craft the complaint and could get into the specific reasons why they were denied service, tying their “offensive” statements to their brand of religion.

I doubt the restaurant could claim making a plate of food is art the way a custom cake designer could. So it is likely different in that respect.

Could actually end up being interesting.