The question THIS LAW interacts with is the CONTENT of the message. If you’re providing tables for a wedding this law wouldn’t protect you. If you were asked to write something specific for the wedding and the content of the request is antithetical to your beliefs, this law would protect you, if you could show that. Not a lawyer, but that’s how I read it.
Now. Is it “right” to do so? I would say in absolutely no universe. It’s morally wrong, it undermines our liberal society, and I have no tolerance for it. My point is that this particular law isn’t about whether someone is a Christian, their race, or sexuality. This decision wouldn’t protect me from writing some basic software for a nazi (others might) but it DOES protect me from building a website supporting them, or writing prose related to nazism, or anything else which would be CLEARLY against what I believe. Please DON’T read that I’m saying that being a nazi is the same as being homosexual, it isn’t, I’m not, fuck nazis.
To get back to your question: as I read this decision, a cake maker could potentially be compelled to make a cake for an interracial couple, but they might not be compelled to make a cake with something like “interracial is the only way to go”