In spite of my overall tendency to criticize Jack's role in the reletionship, his 20-year desire/wish for the relationship to be something other than it was--and staying with Ennis--is a testament to one of his strong, good traits. Since about the time of the setting of the film's opening (mid 60s), we have been a 'throw away' society, and that includes people. I have tons of theories why such occurred, and grew, but fact is that injest's experience is not uncommon. Jack continued to choose to view the 'glass is half full' but more and more, as freedom (not the 'americana' sense but in terms of ability to move places, leave jobs/careers, reduce responsiblilites, etc) and affluence rise, people seem to be unwilling to accept 'half full' and insist all glasses be pretty close to overflowing.
Jack had what few people have, and definitely few gay men. His man, Ennis, was fully committed within the bounds he was able to work within. There was no evidence that Ennis was ever seeing another man, his inner, heartfelt love for Jack showed throuhout their 20 years together. No, Ennis was not the flowers every Friday and 'I love you' every phone call, but there is great comfort and pleasure to know and believe in something that need not be promoted every moment. Ennis was solid. Jack knew this and to his credit knew Ennis was a true blessing and a rare human. Jack's mistake, I think, was the disease of the past decades..the 'you can have it all disease' and while Jack's half full glass was a relationship from Heaven, he imo failed to be grateful and satisfied with Ennis as Ennis, and lost everything...for both of them.