On your way to the chapel there are two NPC's underneath the gated arch. The wedding planner that starts the quest for the Rings, and an npc to her right that describes the ceremony process to you. This npc also has an option to annul your marriage by handing your rings in to them. A divorce so to speak.

One benefit of this is that you can keep signing up for free marriages to obtain a constant supply of marriage consumables (Heart balloons, Wings, 150 Eternity Cakes for your Medium house, etc.)