They haven't discriminated against a particular group per say - they've declined to create a message on a cake relating to an issue supported by a large number of that group.
The Equality Commission seems to disagree.
It is a freedom of speech issue IMO - its not too different to say a London based Arab publication refusing to run a pro-Israeli advert in their publication. Would that be anti-semetic/racist? I wouldn't say it is as its the message/advert they're disagreeing with not simply a refusal to run an advert because the person requesting it happens to be Jewish.
No it wouldn't be, unless they said "We aint running it because you are Jewish".
I do find it odd though that most people supporting the bakers keep trying to equate supporting gay marriage with being forced to "support" bad things.