A business transaction isn't the same as supporting a political campaign. A bus driver taking fifteen Conservative supporters to an event isn't supporting the Conservative party - there is no reason to bring in personal beliefs into a standard business transaction.
Interesting that you mention bus drivers..Bus driver are acting servants of The Company, they have no option but to follow the rules and regulations set out by The Company...it would be The Officers of The Company which set those rules and they do retain the right to accept or refuse a contract as they see fit within the law...When I was an Operations Director for a commercial public transport company we had very defined parameters for what was acceptable to advertise on our vehicles for example, the basic conditions were (words to the effect of) that the company would not condone, support, endorse or be seen to support, condone or endorse any policy or be associated with the aforementioned, or make commitment or endorsement of an illegal act or an act or product that might be seen to be controversial or immoral either directly or by association.
This meant that various requests for hoarding advertising on our fleet were turned down, including ones from Political and Pressure Groups. Some where accepted and then removed at a later time when it became clear that the image, message or subject matter was politically or morally controversial. There was no discrimination in this as it was a blanket policy.
Just because it is a business transaction doesn't mean that you must accept that transaction, particularly if the transaction would mean compromising the ethos and/or business of the company involved.
As long as what's being requested doesn't break the law, refusing on discriminatory grounds (which can include politics, sexuality or race) is not something the law in this case allows.
Same Sex Marriage in Northern Ireland is currently illegal, it is not unreasonable to assume that creating a cake that says "Support Gay Marriage" could be construed as endorsing or condoning an illegal act...whether that act is moral or immoral or whether the act itself is part of an ongoing legal challenge or not is not the point, the Company did not refuse to serve anyone because of their sexuality, religion or gender..they simply refused to produce a product that condoned an action that was not in keeping with the ethos of the Company as defined by The Officers of the Company and is currently illegal in the region in which they trade.
They offer a service making cakes, the second they start a business up for trading they agree to our collective rules & legislation - if they are unable to abide by these rules they should perhaps reconsider if the world of business is for them.
Indeed, and currently there is no law that requires a company to support or condone Same Sex Marriage in Northern Ireland. If they were asked to bake a cake for a Civil Ceremony and they refused that that would be discriminatory, if Gay marriage was legal in Northern Ireland and they refused to bake a Wedding cake because it was for a Same-Sex Marriage then that too would be discriminatory..however the big hiccup here is that unlike the rest of the UK, Same Sex Marriage is not legal in Northern Ireland and therein defines the right of the Company to not endorse or be seen by association to endorse an act that remains illegal in the region in which they trade.
I simply disagree that baking a cake is an action of support to a political view
You are quite right, baking a cake doesn't...but the slogan being asked to be put on that cake does, simply by association it can construed that the company supports Same Sex Marriage...whether it does or not is immaterial.
(I also don't agree that equality is a political argument to begin with) they may not share. They are free to not support equal rights for gay marriage, their business on the other hand is not.
Their business is free to not support a change in the law by association...this means that they can refuse a contract on the basis that the contract may prejudice their business or may associate their business with the acceptance and endorsement of whatever they are being asked to create a product to support.
No business should be forced to support any change in the current law, even by association, unless they want to. Again, Same Sex Marriage is illegal in Northern Ireland and until that changes they should be free to not associate their business with Same Sex Marriage.
To be clear to everyone ( I realise Elmarko probably knows this), Personally I fully support the position that Marriage should be available to all, no restriction should be placed on a consenting couple (or even more in my opinion) who freely wish to express themselves and their commitment to each other through marriage. I also support the position that Civil Partnerships should be freely available to all in the same way. the only restriction I would place on these is the requirement of Consent of all parties who wish to make such a commitment, and their right to withdraw that Consent at any time without prejudice or punishment. 
