I see the beauty of their marriages. Jack and Ennis had the freedom of an "all boys" week, fishing, hunting, and getting drunk. Their wives could be easily excluded. Their 20 years affair, lasted so long because of not only of their love, but the fact, these men can use their sex as a cover. The once or twice a year "male bonding" aspect of their relationship.
Now, Randall could have put a monkey wrench in that alone, being male closer to them, like them. Unless Jack could come up with a reason for Randall not going along with him to Brokeback. Jack would have to come up with a good lie, or just right out tell him, as a understanding in their relationship, he let him go once or twice a year to someone who needed him just as much as he did, tell the truth and take it from there.
Lies, compromises, concessions. How far can love take someone? We seen in this relationship with Jack and Ennis. Lies, compromises, concessions. How out of realm of reality with these men. Could this scenario have been possible?