In the film, for me, you have to watch Ennis's actions and reactions. From the first summer to the time he got the returned postcard. Ennis was a character you had to read the signs, a little more than listen to his words, I may not be clear.
You see him open up to Jack in little ways, ways you have to watch for despite of his declaration of not being "queer". Being stunted emotionally by his father, the lost of his parent, being shove from sister to brother, and finally alone, no one ask him about how he felt, what he wanted, so he kept quiet.
He loved Jack, Jack was the only person he really loved. You have to look at what happened in his life, the events in his life to see that Jack was it for him. Everything in Ennis's life centered around Jack. The very sad part is that he couldn't reconcile himself to it, until it was too late.