Who told Ennis what there is in Mexico for guys like Jack?
Literarily speaking, it must have been Larry McMurtry. But, in the way that I read the short story weeks before I saw the movie, I believe what Ennis had heard about Mexico had nothing to do with Jack going there to have sex. What he had heard was that queers got killed in Mexico, too, IMO. I have met guys from Mexico who would confirm that fact and that only "bottoms" were considered to be "queer" in same-gender sexual contact, those who were tops could claim to be "straight."
Annie Proulx never wrote in her narrative that Jack even went to Mexico. He only claimed he had been after Ennis asked "Been a Mexico Jack?"
The only reason that Jack even brought up the subject of Mexico in the story was he wanted to meet someplace where it was warmer than the high altitudes of Wyoming. Jack was getting older, he was 39 you know, and the fact that he had all of those bone injuries from riding bulls, he apparently suffered from arthritis and rheumatism, too.
(My older sister's 2nd husband was a rodeo bull rider a long time ago. Many of his current health problems are related to his rodeo days.)