Soldato
From the discussion with the manager, etc. it doesn't seem its something they took lightly or purely down to to (irrational) homophobia. No different really from them deciding to turn down producing another cake that had features they couldn't reconcile with their beliefs i.e. if someone wanted something that was blasphemous to their religion.
That wouldn't be "no different" because, in that case, the requestorof the cake would be the discriminatory one. So completely opposite