Morning, lily, jings, jane, et al.
"Is failing to display religious posters showing disrespect for Christian beliefs? If I were politely to refuse to advertise a gay pride march would that be imposing my views on others? Or discrimination, in a legal sense?"
No, lily, that wouldn't be showing disrespect for Christian beliefs. However, it would be a strange thing to do if it was your job to display posters for all kinds of events.
I think tolerance of others life views, approaches to life, in the public sphere of his chosen daily work, is what the baker lacked. He was asked to do something he does for a living, to provide a cake. He doesn't have to agree with the purpose of the cake (cakes with purpose! oh boy!) to just supply a customer with the cake they want.
If he had simply been in the habit of making cakes for friends out of the kindness of his heart (so it wasn't his job) and a friend asked him to supply the same 'gay pride' cake, his saying no, he'd rather not, would have been fine.