There is no doubt that Jack and Ennis were deeply in love with eachother. Even in the last scene together you see the sadness in Jacks face as he gets ready to depart. Ennis will never forget about Jack. I think most of us still carry part of our first love with us in our hearts. But I truly believe that a person can love more than one person. And often there are different types of love. Love of partner, love of family, love of friendship.
My mother, who is often a very insightful person, has likewise stated that it is possible to know true love with more than one person, indeed, that it is possible to have more than one soul-mate. This is likely to be as true for Ennis (and Jack, for that matter), as for anyone else. The question is, would Ennis ever give himself the freedom to share his heart with another after Jack's departure? If he could find the means to do so, this could provide some measure of happiness for him during his remaining time on earth.
David, I also like how you affirm the different guises that love takes. We often place a lot of focus on romantic love (especially when we don't have it!), but other forms of love can be rewarding in their own right and are equally important. I myself hope to find romantic love one day, but I am cultivating love for my fellow beings generally at the same time that I nurse that dream, recognizing that each person is a unique manifestation of spirit who has something of value to offer the world. Ennis has love for his children, and this could serve him in rendering his heart more open to others in general, whether or not he encountered romance again. It is in giving love that we receive love.