I've said it before on here; you wouldn't ask a Jew to sell you ham. What I think a lot of people miss is the the fact that there actually are observant Christians. Almost since it's inception the US has been a "Christian Nation" but not a particularly doctrinaire one. Because so many non observant Christians exist it's easy to forget that some folks do take their faith seriously. Reynolds v. United States protects the general public from the consequences of individual faith, but doesn't mandate that people of faith are obliged to act against their religion while operating within the law. If it were a different question, say hiring a qualified gay cake decorator vs. an unqualified heterosexual cake decorator I would be on the side of the qualification over orientation. But it's not. The guy chose principle over profit. Whether his religion is wrong or right ain't up to us to decide. Principle is rare enough in this day and age, let's not actively try to destroy it.
And it’s a particular service too. It’s not like he won’t sell them other goods, just not anything that has to do with marriage.
My take as always why patronize a business who has openly declined providing you the service you need. Total nonsense.