The key issue here is that these gay activists happened to unfortunately choose a Christian activist bakery who 'took a stand' based on their Christian beliefs.
Had I been the owner of the bakery, I would have simply stated, "The text on this cake is political, and we do not produce any partisan materials."
This is effectively what bouncers do when they want to keep certain groups out of bars.
If this had been a straight pressure group (say the Coalition for Marriage) who had similarly requested a cake that read 'Marriage = one man / one woman', they would have received service, therefore, the actions of the bakery can be seen as discriminatory.
Perhaps thats what needs to be done...a gay-freindly bakery should be chosen at random and asked to produce such a cake. If they refuse the commission then...
...actually, I don't know what that would prove.