As Sawyer might tell you, there's always Full Throttle.
Maybe so, but that was incredibly short-lasting. And the game wasn't the longest or deepest either.
The final truth is, games are incapable of showing mature, long lasting relationships. Where you constantly are with, or just lay down, have a nice conversation, or just sex with your partner. Fu**, i want to have a real in-game wife/husband.
The game doesn't have to show a long lasting relationship to show a mature one.