I don't believe that. While it is true and obvious that Ennis loved Jack more than he loved Alma, I don't believe that he was without love for her. IT just wasn't the same. He loved her as much as he knew how to at the time. He just stated that he was concerned about her being satisfied enough when they were together, to learn and DO what would make her happy.
I do believe that he has been occasionally interested in women, and has already stated that some catch his eye occasionally. Throughout the whole getting to know Ennis, through BBM, book, movie, and this Laramie Saga---he has never said he didn't love Alma, just that he never loved her like he did Jack, and now like he does Ellery. But I do not believe that the only reason he was ever with Alma was to hide who he was. HE may have not quite even understood who he was, but he wasn't hiding.
I think there's a difference between loving somebody and being in love with somebody. At least in spanish we make that difference: querer a alguien vs. estar enamorado. It's not the same thing.
IMO, Ennis loved Alma, but he wasn't in love with her, not even in lust. He married her because he thought it was the right thing to do, and also to conceal his homosexuality from society and his family, I might add. While I agree that maybe Ennis to some extent didn't know who he was, he knew very well what turned him on, especially after being with Jack. How could he not? Ennis was in denial out of fear but as soon as he saw Jack, he knew what he wanted. As we all know he was afraid of being killed for being queer. And part of his behavior has to do with his childhood, with his father taking him to see the corpse of Earl. IMO, if Ennis had grown up in a city and had more education, he'll be 100% gay, just like Ellery (Or at least 99.9%, since it's a fact that there are gay men who think some women are attractive but wouldn't have sex with them).
I've gotten to this conclusion because of his passion towards his male lovers. Ennis is extremelly passionate when making love to a man. There's a scene in the movie that speaks clearly about this. When Ennis is packing to leave to the mountain to "go fishing" and Alma makes the comment about the job opening in the power company, Ennis looks like a caged animal desperate to be set free into his natural habitat. He was also shown truly happy with Jack, just as he is truly happy with Ellery, and pretty miserable with Alma. And he used to penetrate Alma by the ass. That's in the short story and the movie.
But Ennis out of habit and of need learned to like women to some extent. But it is not his nature. It's the same thing as with Dupree, but the other way around. He spent a great deal of time with women. First Alma, then Cassie, and had learn to like them, and to find beauty in them, and in being with them. But that's doesn't make him 80% gay, IMO. I think Ennis is 100%, maybe 99.99% but I'd give him a 6 on Kinsey scale.