If someone went into a cake shop owned by a Muslim and asked them to print something blasphemous or directly against a principle then how would you see that being played out?
Islam has even harsher opinions towards Gays than Christianity, so if we leave the request, the cake design, the same and have a Muslim refuse to make it then they would have broken the law in exactly the same way.
If they were asked to produce a cake that was specifically inflaming to Islam, say a cartoon of Mohammad in bed with Elton John, etc, or even if we put Jesus in that cartoon and go back to our christians, then the case is more complex, because what they were asked to produce would have been quite hateful and possibly hatefueled.
According to the bible, which I do not believe, homosexuality is a sin. So are a lot of things, so does this baker refuse to produce wedding cakes of cohabbiting hetrosexual couples? Or for divorce parties or divorcees remarrying? Or for liars, or people who have sworn at thier parents?
Protestant theology does not even admit a hirearchy of sin, so a lie is just as bad as a single sex orgy.
What I'm trying to show is that the law is correct because these people are using their religion to justify thier unreasonable hatred. If it was a case of sticking to the rules as laid out in Leviticus etc, then they would even refuse to serve women during thier menstration.
You can not discriminate on only a few principals, on Gender, sexuality, race and religion.
A Baker can't refuse to make a cake for a specific wedding or christening. They could refuse to do wedding cakes completley but they can't exclude just one part of the wedding cale market.