This man is an artist, not just a cake decorator. Should an artist be forced to waste his talent on something that violates their conscience?
Lazy excuse. A cake maker who offers a service to make wedding cakes has to abide by anti-discrimination laws when deciding who is entitled to pay for his service or not. It doesn't matter whether you pass it off as them being an "artist" or not. This is not simply a matter of free expression - it is a matter of denying service on unjust grounds.
Not a slippery slope at all. To ask someone to incorporate something into an activity that is against the law is not right. The law in this case is the Law of God, which overrides any law made by men.
For him, but not for many, and laws exist that prevent people who operate businesses from using their beliefs (religious or otherwise) as a basis to discriminate against individuals and/or groups by denying them a service, in accordance with anti-discrimination law.
Deeje, you know very well that if this individual was instead denying service to an interracial couple on the basis of his religion, you would not be defending them. You would also not be defending them if they denied a service to someone based on their political views, despite the fact that said views could be just as strongly held as any religious view. This isn't a matter of allowing this man to have his beliefs - it's you calling for special permission for religion to use businesses to discriminate against clients and push their agenda on people.
At least be honest about it.