If the halal deli usually serves bacon sandwiches, then I'd expect them to serve anyone that stopped in and ordered one, so long as the customer ordering one wasn't a nuisance or health risk to others in the restaurant (the whole "no shirt, no shoes, no service" thing).

The couple wanted a wedding cake, which the baker does for others.  He is refusing to do one simply because they are gay.  That's the issue....and why your "bacon sandwich" analogy doesn't really work.