Because freedom to express ideology is not a protected right in the private sphere (thus FR can zot libs) vs, amoral aspects as race, and SCOTUS has wrongly chosen to equate homosexuality as race or ethnicity. Thus since a private entity cannot actually discriminate against people on the basis of race, then neither can it refuse services, even special works of art if such are offered, based on being part of a sexual orientation "ethnicity." This is the tragic immoral consequence of of SCOTUS going to war against God.
However, since the likes of FB can censor those who express ideology that is associated with a non-PC color or race (like removing Dr. Seuss books) under the broad category of Terms of Use as "hateful," "offensive," etc.) then since homosexual marriage is an expression of ideology then one could try to justify refusal on that ground.
Be wise now therefore, O ye kings: be instructed, ye judges of the earth. Serve the Lord with fear, and rejoice with trembling. Kiss the Son, lest he be angry, and ye perish from the way, when his wrath is kindled but a little. Blessed are all they that put their trust in him. (Psalms 2:10-12)
Sexual “orientation” is a choice, not a trait like skin color. It’s a choice every bit as much as,jointing one religion or another, or no religion is a choice. So it’s odd that one choice is elevated to trait while the other isn’t. Religion “should be” every bit as protected as skin color is protected agaisnt discrimination if they are going to protect an immoral choice like homosexuality as a trait
The left can’t even be consistent. One evolutionist describes relgiousness as stemming from a virus (the evolutionist that argued agaisnt Michael behe- can’t recall,his name right now), and other folks falsely claim that there is a “gay gene” and that people can’t help being gay- this they argued is why it needs constitutional protection like skin color gets. But if the left argue that religion or folks have a virus, then this means “they can’t help what they are either” and should be protected too
Of course it’s as false to say thst religious folks have a virus as it is to suggest that gay people,have a gay gene and can’t help their choices.