Jack Phillips does stand behind his own principles. He said "'I don't create cakes for Halloween, I wouldn't create a cake that would be anti-American or disparaging against anybody for any reason, even cakes that would disparage people who identify as LGBT. Cakes have a message and this is one I can't create."
But he responded as an artist. Lets not pretend his cake would identify him as openly supporting gay marriage. He may feel that way, granted. But your comment that "They didn't want to decorate a cake with messages that went against their principles and beliefs" leads readers to believe more than that.
I don't know about others, but I can admire someone for their moral stand, yet disagree with them about it at the same time. People want a Halloween or Gay Wedding cake, offer to make them a cake. You can refuse to put inflammatory insults on cakes, but you should still make them a cake. If they wanted swastikas on a cake, say "You'll have to do your own message, but here's your cake."