I was somewhat shocked to hear the MOVIE Ennis say, "
Boys like you," to Jack.
IMO, the only reason that Jack brought up "Mexico" in the first place was he wanted them to meet were it was warmer in the spring, summer, and the fall than it was in the high altitude mountains of Wyoming.
In the book, Ennis only asked Jack if he had been to Mexico. I don't think Annie Proulx was claiming that Ennis believed Jack could be sex with a guy when he wanted it anytime he wanted it in Mexico; I think that he had heard queer guys got murdered in Mexico. And, understanding what kind of homophobic man Ennis's father was, Ennis might have heard that from Daddy Del Mar himself.
All of Ennis's homophobia was rooted into what he saw when he was about 9 years old and his father made him see the beaten with "the tire iron" and mutilated body of Earl, the partner of Rich and the member of the old ranching couple near Sage, Wyoming.
Jack's private life away from Ennis was, IMO, none of Ennis's business since they did not have a covenanted relationship in that Jack would not have sex with any other man.
"Hell yes, I been. Where's the fuckin problem?" Braced for it all these years and here it came, late and unexpected.
The above quote is from the book. I believe that Jack "lied," claiming that he had been down there; when really he used "Mexico" as a metaphor, so to speak, to admit the fact that he had been having sex with other guys besides Ennis. Jack had plenty of opportunity when he was on the road between Childress, Texas and Lightning Flat, Wyoming and when he was working for the farm and equipment company in a managerial position of sorts (the books' version is different from the movie).
It is just not plausible for me to believe that when Jack was rebuffed by Ennis when he showed up unannounced after Ennis's divorce that Jack would redline it all the way to Juarez, Mexico just to have sex with a male prostitute because he didn't get any from Ennis.
I have encountered "straight" guys who went looking for sex with gay guys after their wives and/or girlfriends would not put out for them when they were horny . . . but they would not have driven more than 10 miles to do that.