OK, so Jack tried to pick up Jimbo, then went to Mexico, then presumably responded to Randall's overtures; meanwhile Ennis was having sex with Cassie in the trailer, his wife having divorced him. In other words, although once Alma was gone and Jack's marriage to Lureen had become something, he says, that they could do on the phone both of them could have been physically faithful to the other, neither did. I suppose I dislike the idea so much because I like the thought of them realising the other one is the only one and since they can't be together permanently at least not finding room for anyone else when they can't be together. Unrealisitc, I suppose - certainly unromantic.
You could read all this as each attempting to find a more satisfactory relationship but failing no matter what they try, perhaps, and so showing the importance of what they have together - but this feels a bit desperate.