I do believe in freedom of speech and that it should be protected and the it is quite likely that the gay couple sought out trouble for attention which they should not have done.
However, he still doesn't have a right to deny service (or suggest alternatives etc.). As requiring someone to complete a service for payment is not the coercion or prohibition of speech or expression. Ideally, we wouldn't have any reason to have this conversation if the couple had sought to avoid trouble or if the baker hadn't held preference or whatever variable you wish to change.
His service / product is the creation of his artistry in the form of cake, and they were not asking him to halt his creative expression, they were asking him to continue it for a particular purpose, thus there is no right being violated
also displaying an image of blatant situational bias does nothing to help your credibility