There religion is a protected characteristic for both.
But Orange marches are a political act, not a religious obligation.
(e.g. a Muslim taxi driver who refused a passenger with a guide dog because of his religion was held to have discriminated)
But here there's an obvious safety issue at play. Also, taxi drivers are supposed to accept any passenger unless perhaps they are drunk or abusive. A bakery, like most private businesses, is not obliged to accept any job.
In your example I would imagine the right to refuse to ice the cake would win as the other side is clearly acting in provocative manner.
But couldn't you say the same here? From what I know, this bakery was well known to be run by conservative Christians. Belfast is a pretty big town. The customers could easily have chosen one of any number of other bakeries. Of all the bakeries in all of Belfast, they happened to choose this one?