Fiona, having been born in Oklahoma in 1966, I can't speak fully about 1963 Wyoming culture. Only thing I can add to your post is that the words "I love you" were never, ever spoken in my house, up until 1984, when dad was driving me away to college, and that was the first time in my life that I said it to my mom, on the front porch. She looked shocked, but happy, to hear it, and she said she loved me too. I think there are certain, more stoic, families and places in America, where words just get left unspoken by choice. It's unthinkable to say such things that might get you into trouble or get you in deeper into a situation. It's just understood from day one, that you don't say certain things.
I would have to agree with Sue/Katie77 that those three words between Ennis and Jack that were never spoken in the film, were indeed expressed in other ways. All it takes is one look between two people to tell that they're in love forever. And we, as viewers, got to see that look time and time again, starting from the bar scene, to the Jack and filly scene, to the Ennis looking up at the sky scene, worried, and so on and so on. Words are just words. They come and go, and can be taken back, and apologized for, and regretted. They can be true or false, or half-lies, or half-truths.
But the eyes never lie...