Does every discussion of this issue have to dissolve into semantics? "But, what if the dude like hates Chinese, and the cook is dating a Chinese chick? What then?"
Law. The law. The law says you don't get to deny someone service because they're gay. Just like the time I had to take down that sign on my lunch counter that said "Whites Only". Get it? You might hate black people, but they have the same right to life, liberty, and the pursuit of happiness as every other citizen. And discriminating against someone for being born gay is exactly the same as a Jim Crow car. If this baker's view of the world is so distorted that he thinks writing "Stick it in my hairy bunghole" (I assume a traditional gay wedding expression) in pink and chocolate frosting will somehow harm him, I for one am overjoyed at the idea of his death due to starvation, and subsequent exit from the gene pool. The law occasionally exists to protect people who have no other recourse to protect themselves, and if you feel differently, you are one of the bigots who made Selma the lovely place it was in the '60s, whether you mean it or not.
I think it should be illegal to force someone to make food for retarded or elderly people. After all, they're a burden on society. And don't get me started on Jews, natives, and Italians.