There has been much commentary on the Gareth Lee v Ashers Bakery law case, some links are provided on the TA website to which may be added Martin Salter's reflections on cake, conscience and commerce. Not having read all of these but a number, I am still not persuaded by the first part of the judgement claiming that the customer was (indirectly) discriminated against on the grounds of sexual orientation. Evidence for (direct) discrimination on the grounds of political belief, as argued in the second part, seems to me much stronger.
One point of the controversy deserves maybe more attention that it has received. It is the question what sort of product this cake is. The managers of the bakery considered it a vehicle for a political slogan with which they would be implicated by making the cake. That the cake expresses a political conviction can be granted but the Judge rightly observed that no consideration seems to have been given to alternatives which could have created the necessary distance between the bakery and the political conviction. While not mentioned in the judgement, use of neutral packaging might have been sufficient to prevent any association of the bakery with this political view.
The Judge seemed more inclined to consider the product just a cake, one in the generic category "cake with graphics and writing on it". To deny such a cake to one customer but not another is unlawful discrimination. This makes sense to me. If baguettes are sold to some customers but not to others we have a problem.
So the question really is whether one cake with writing on it is just like any other generic cake or whether a cake with the slogan "support equal marriage" on it is a specific cake, one which had never been sold before to any customer and which the bakery does not wish to make for Gareth Lee or indeed for any other customer under the sun. I do not think the answer to this question is obvious; I am surprised that apparently it has not been explored explicilty.
The situation would be similar, if a bakery offered "wedding cakes" with a groom and a bride figure on the top of the cake, saying this is our "wedding cake" - take it or leave it (or, if you are a gay couple hitching up, club together with a lesbian couple, buy two cakes and change the figures.)
A related question may be weather a bakery would be free to provide "baptism cakes" but not "bar mitzvah cakes" or "bar mitzvah cakes" but not "bat mitzvah cakes", or whether a bakery that offered Ramadan-themed cakes would be obliged to provide "baptism cakes" as well.
It is a fine line between the freedom of a baker to offer the products they like and the right of customers to buy whatever another customer has the right to buy and the line goes through the specifci/generic distinction, as far as I can see.
If Gareth Lee had come into the shop, saying I'd like this cake over there for an anti-homophobia event with my gay friends at QueerSpace and if he had been unable to purchase a cake, it would have been a clear case of illegal discrimination. The fact that a cake with a particular design had to be specifically produced prompts the question here.