One involves simple race, and the other involves action. If the Baker had refused to serve gay people, then it would be a closed case, but this Baker did not refuse to serve the people, it has already been proven that they were served graciously all along, however they did not want to be a part of a ceremony that went totally against their moral conscience.
First, it's not gracious to accept money in exchange for a service.They weren't doing the gay couple a favor. They were happy to do it, the reason they were in business in the first place to to accept money for weddings.
Your argument falls flat, could they also decide to not serve a wedding of a mix-heritage couple? If not, why not? How is that different? What if the couple getting married were Hindu or Muslim or JW?
How is it moral to serve some occasions for a gay couple (who you know are doing acts you don't approve of) vs. getting marries, where those same acts will occur?
If being black isn't a choice and being gay isn't a choice, what is the difference?