Within the confines of this argument, it would depend on if the creators disagreed with the fundamental morals Christianity, or if they merely disagreed with the principles practiced by this specific branch of the religion. There can be a world of difference between The Church and a church, and that aside, it's illegal (in the US) to discriminate based on religion. It's not illegal to discriminate based on the actions of a group, religious or not.

Could you explicitly turn away all Christians? No. Could you turn away the Westboro Baptist Church? Yes. Morals are defined inside the law if you want to stay in business for very long.

As you've seen from the article, this is not "scary" in any way because most businesses will go for the money over compatibility with their moral views.

Also, advocating or even practicing exclusivity is not a precedent as it has been going on for centuries, in thousands of different forms.

Some of those most people disagree (e.g. there was a time a black man would not be served in many restaurants) and some of those people accept as completely valid (e.g. I'm sure there are many country clubs I'm not eligible to join).

Yes.

Just don't complain when all the religious customers go away.