In Ar Tonelico: Melody of Elemia, young knight Lyner Barsett must insert his life-extending crystal into the back of Aurica Nestmile, an artificially-created being known as a Reyvateil. It's his first time but don't worry, he'll be gentle.
Another triumph of localization, the North America version of Ar Tonelico and its two sequels were just as raunchy and inappropriate as the Japanese originals.
The series plays on the relationships between humans and the Reyvateil, artificial beings with the ability to channel sound into magic. In order to do so they must form strong bonds with their human users. This is accomplished through conversation to an extent, but more effectively through either sliding crystals into the Reyvateil's installer ports, or diving into their subconscious—entering them—and helping them craft more powerful songs by working through the twisted maze of their psyche.
The latter is actually a rather interesting concept. Each Reyvateil's consciousness is a multi-leveled stand-alone adventure, a game within the game. Helping these artificial women overcome their issues leads the player deeper into her mind. It's a highly entertaining form of character development.
And one ripe with opportunities to make voice actors moan inappropriately.