Yes, I think that Jack understood that to admit to a more sustained relationship with another man would hurt Ennis. I think that Jack himself probably felt ambivalent about his relationship with Randall -- at least, before the Last Confrontation. Why did he pursue it then? I think it was because of the growing uncertainly of ever getting Ennis to commit to a Sweet Life. Ennis just had to say "yes" and everything else would not have mattered an iota to Jack. Perhaps Jack "justified" the "lie" by his realization that on the scale of his affections, nothing else would come in the way of his love for Ennis, even as he grew increasingly frustrated of what he thought was Ennis's refusal to take their love to the next level.
Although I agree with you regarding Jack "settling" for Randall because he felt Ennis was out of reach from the sweet life, I disagree with you regarding Jack's lying to justify his affections.
I am not sure what you are refering to in this. I thought the lies were to Alma and Loreen, to society in general, and to each other.
Perhaps it would be helpful if we itemize the lies.
Specifically, athena was referring to Jack hiding his indiscretions with Randall (which, by the way, is not definitively stated in the movie, but potentially mere speculations on Ennis' part). And also the lie by omission regarding his ventures into Mexico.
The second lie, by omission, depending how many other times since the divorce scene, may be justified by Jack in his own mind, due to the humiliation he suffered after the divorce. It was like payback, and we can see his increasing anger in their meets...finally coming to the surface at the final confrontation.
The first lie, as someone already pointed out, could be for two reasons, one was Jack's fear that telling Ennis there was another man may get Ennis jealous and he didn't want to lose Ennis. Since Ennis was so angry about Mexico, he was correct in his estimation of Ennis. The other reason was also from the same knowledge, that he knew Ennis would be hurt by his infidelity and in his own way, he was hiding the fact to protect Ennis from getting hurt.
It just occurred to me, that there may be another reason. If Jack admitted to Randall, the logical question would be the one he asked Ennis, "You never found another to marry?" "You ever found another for the sweet life?" "What are your intentions with this fellow, Randall?" It puts their relationship under the spotlight, and Jack may not have been ready to lose Ennis and settle with Randall. Indeed, he may not have been sure about Randall, or worse he knew Randall wasn't going to settle on the Twist Ranch with him either. Afterall, Randall was just as happy with his nice life in Texas as Jack was. So the lie about Randall was Jack's way of having his cake and eat it, too.
The lies to Alma and Loreen were bad enough, having been forced on them by society's homophobia.
The lies to each other are simply sad, making a difficult situation even worse.
Jack was AFRAID to be honest with Ennis because he was afraid of losing him. Out of the original homophobic fear, more fears arose to weave a web that trapped them both.