说好的白头以命为期约好的陪伴永垂不朽
[shuō hăo de bái tóu yĭ mìng wéi qī yuē hăo de péi bàn yŏng chuí bù xiŭ]
It conveys a deep longing for love everlasting till death parts, with mutual agreement for company until eternity which has an element of romantic idealism and commitment that lasts forever. A more poetic way to express a solemn vow of love and companionship to the point of immortality.