Well, technically ANY game that has plotlines and feature some sort of marriage may count. So I'd have to say Majora's Mask.
Kafei and Anju's marriage at the end of the game!
If you mean pivotal to the actual gameplay, I'd have to say Fire Emblem: Genealogy of the Holy War! --- Currently playing: SMT IV, Ys Origins, Mario and Luigi: Dream Team, Castlevania III: Dracula's Curse, and Batman: Arkham Asylum