hmm, not sure what to think of this. On one hand businesses should have the right to choose what business they take, but on the other hand refusing customers due to their sexual orientation is not a good precedent.
I guess really it should depend on what the details were. ie. if they wanted a custom made cake then he should be well within his rights to refuse, but if they were just buying a regular cake from the menu or shop window or something then refusing that should be grounds for a discrimination case.