When Jack mentioned Mexico I don't think it was for anything more than a warmer climate.
I don't think he knew that Ennis knew what went on there.
As far as women, Ennis married Alma cause that is what he thought he was supposed to do.
As far as Cassie, she interjected herself into Ennis life. She came after him.
When Ennis ask Jack about moving August meeting to November:
"do you have a better, idea,"
Jack: I did once.
Ennis: You did once, you mean Mexico, Jack Twist, cause I heard what they got in Mexico for boys like you".
This was not a passing comment, but a statement of someone who has knowledge. Ennis knew exactly why Jack went to Mexico for, and this must have been killing him inside.
Look, all I want to know, if anyone felt the same I did when I watch the movie (for millionth time), Though Ennis had a wife, and then Cassie, he went to no other man, and Jack was his true, and only love. Jack who also had a wife, still found the need to be with men, but they weren't Ennis whom he loved.
The way they lived their lives when not together, was it something that was of a wedge between the two? Women were safe no threat, but another man was a real problem, it hit right to heart of the relationship.
Ennis's possessive love for Jack, but his deep seeded fear, kept them from that "Sweet Life", but it also cause that long duration of time between meetings, Jack lived by the rules Ennis set. But Jack was not a pope, he acted on his needs.
Ennis could take him screwing around with women, but men, no way, that could get him killed. Does this goes to control of Jack, or was Jack the one with control. He knew exactly what to say to Ennis, when to say it or not say anything at all.