I mean, in the context of this case, they had no problem providing a cake for them. Where it gets dicey is the "art" part of the industry. Some people may not feel that way about cake decorating but certainly anyone in that industry probably views it as some kind of artistic expression and thus decorating it is more like a commissioned work
I'm not sure where your argument of "It's your job, you have to do it" comes into play in this case.
Maybe it's different in Ireland but the context changes when it's a public business that's open to the public that generally makes a product and puts what the customer wants on it. Exceptions based on bigotry shouldn't be tolerated. It's common sense that the sentiments of the cake are those of the person who purchased and asked for the cake and not the baker. So saying "I don't agree with it" is kinda moot.
It is discrimination because the desire to not write it is 100% bigotry. People don't see it as discrimination because conservative fuckhead racists have screamed so loud that gay marriage is a political issue that some folks take it as truth similar to how loud idiots declare climate change is a hoax. The way this issue is framed is just shameful because a lot are buying into thay gay marriage is a political issue.
Thank you. Holy shit.