and taking the short story purely on its own merits .. it is clearly Ennis that is completely in love, and it is Ennis for whom there is one man and one man only. Ennis was Jack's first choice, but he had runner-ups. The other element of Story!Jack that comes through much clearer is that Jack moved out of Wyoming and went rodeo chasing in Texas, not contacting Ennis until long after Ennis had given up on him. And it led me to believe he had rodeo thrill seeking, picking up rodeo clowns, and finding a meal ticket on his mind before he had Ennis on his mind. He didnt get in touch with Ennis again until he had had a few years of pounding the rodeo trail, while Ennis wondered what happened to him.
I agree about Ennis being Jack's No1, but there being runners up (I like that turn of phrase, Louise!). Which is one of the reasons I can believe Story!Jack taking up with someone else and saying to his father he was bringing another bloke to the ranch. But have a hard time thinking about Movie!Jack doing the same. The Jack in the story comes across to me as being self-centred and boastful; not likeable at all. Sending the postcard after four years had the impression of being something he did once all the other possibilities had dried up. Whereas in the movie it felt like he, too, had missed Ennis desperately, and he sent the postcard eventually when he just couldn't stand the pain of being apart any longer. And there are loads of other instances where the characters, I think, are different.
So, like Leslie said, the story and the movie are completely different entities. And I agree with the screeplay being called the first fanfic, too.