The posting of a letter to the editor is not a business transaction - and I beg to differ with you about the ad space.
That being said - as far as a normal retail customer:
When business owners hang up open signs, whether literally or figuratively, they have a responsibility to treat all customers equally under the law.
State and federal civil rights laws prohibit discrimination in places of public accommodation for reasons of race, color, national origin/ancestry, sex/gender, religion/creed and disability (physical and mental).
A quick Google search shows that ads get refused all the time. SodaStream can’t buy ads at the Super Bowl and others
Now they are being required to treat all EVENTS equally. These are not marriages. Civil rights does not mean that nonsense gets dictated by government decree. It is like Abraham Lincoln said. If we call a tail a leg, how many legs does the normal dog have. Well it isn’t five.
They make Christian wedding cakes for heterosexual couples. They do not make fornication cakes for queers. Got it?