this poem is not just about love between a man and a woman, but also a man and his country. From what i remember, i believe Neruda was exiled from his home country, which he loved and was very weaved in politics with. so, he's saying 'if my country forgets me, then i will proceed to forget it too' its a beautiful, patriotic poem. about love and loyalty.
not just about a man and a woman, but it can be applied to a man and a woman as well. there are just so many depths to his poetry.