I don't think he knew (or more to the point understood) what love was until he got the postcard. For me, the life lesson that Ennis learned was not about his sexuality, but about loving and being loved. He never knew what love was, or supposed to be, until he lost it. That's why I feel the end of the story, while very sad, left me with a feeling of hope...for him, and for all of us. That's what I thought he meant by: "Jack...I swear..."