There have to be precedents for speech acts undertaken with the assistance of a third party, where it is understood that the third party is not the one making the speech, but merely the one physically inscribing it on a medium (a cake, a shirt, a sign). If it is legitimate for a custom tee shirt company to refuse to make a tee shirt with e.g. profanity on it, the argument could be made that companies have the right to decide what they can render using their resources even when it's somebody else's speech. However, this would only take the bakers' complaint so far, because the baker presumably wants to refuse to bake a cake for a certain type of customer, not just refuse to bake a cake with a certain kind of speech. For example, he would bake a cake that said "Chris and William" only if Chris was short for Christine and the clients were a heterosexual couple. I don't know if there is legal precedent allowing that kind of discrimination, based on the identity of the client rather than the specific message.