This is definitely a complicated question of how many lies were told, and the reasons behind the lies.
I agree that Jack 'withheld information' from Ennis--to protect him. For example--he didn't tell Ennis what Aguirre said about the two of them 'stemming the rose'. Jack also didn't tell Ennis about sleeping with other men, both before and after he met him, as well as in Mexico. This would have devastated Ennis. In their final 'goodbye' Jack lets it slip out--about going somewhere 'warm--like Mexico'. Upon hearing this Ennis confronts Jack by asking if he has been to Mexico (inferring for sex with men). Jack is pissed, at Ennis cancelling their August get-together--and says that their relationship has become a 'goddamn bitch of an unsatisfactory situation'--that he can't 'make it on a couple of high altitude fucks ', etc. So he says , in anger, 'yeah--I been to Mexico'. ^This is the first time Ennis is told point blank that Jack sleeps with other men. This must have floored him--and he says out of depression and overwhelming sadness 'why don't you just let me be?'
As for the 'ranchhand's wife' that Jack says he has been with, and may get shot over, by 'her' husband or Lureen----to me this was a made up story. Jack wasn't sleeping with any woman, (save Lureen from time to time, if that). He said he 'needed' sex with men. In the movie they allude to the idea that Randall and Jack hook up--and outside the dancehall, Randall invites jack to the cabin, etc. I don't know if I buy this either.
It was added to the movie--and is unsettling to me. Perhaps Jack has started up an affair with Randall, since he sees Ennis so infrequently. Jack figured in mid 1970's that Ennis' divorce meant that they would now be together. When he learned from Ennis that this is NOT gonna happen--he leaves and cries on the long drive back. His expectations were blown outta the water. He is floored.
In Ennis' mind, if he truly believed that it could work--that he and Jack could have lived together as partners, he would have done it. He knew this was an impossibility. He , himself said he wouldn't be surprised if his own father wasn't involved in the brutal killing of the older gay man in the ditch. Ennis must have been terrified of his father--and bringing Ennis ,as a child to see the dead body was terrifying to Ennis and left an indelible fear in his heart.