I don't recall the owner of the bakery advertising he made gay "wedding" cakes.
Yeah, the law is the law. The coffee shop owner opened himself up to a potential lawsuit the moment he mentioned the customer's beliefs. It is standing precedent for case law that denial of products, services or civil entitlements because of religion is illegal. If the coffee shop owner had confined his criticisms to non-religious elements, he would be in the clear. The right to deny service to anyone has an unspoken addendum, "...legally deny service". One may not deny service to someone because they are Christian or any other religion.
I have been told by people who know that intention is 9/10ths of the law. The coffee shop owner demonstrated his intention to deny the customer service based on an arguably illegal premise. 'Dodged a bullet if the Christian doesn't sue. He has a strong case, IMO because of the his public statements. The question is whether it is worth the investment to pursue it.
Where is the ACLU when a Christian is mistreated based on his chosen beliefs/doctrine? Out playing golf or protecting transsexual Communist child-molesters no doubt.