Hi I have thought a lot about this story and I think that the whole relationship this amazing love story all branches out from their need for each other. Not just the sex, I think that was a bonus like I have said before, it is this over powering need they have for each others company. The freedom to talk about everything and nothing, or not at all. To sit around and drink with the one person that means more to you than life itself.
Jack goes with other men not so much for the sex, but with another man he can pretend that he is laying with Ennis, he can put an imaginary bag over the guys head and envision his lover.
The sex could be a grand and as real as he wanted it to be, he lived in his own fantasy world most of life. He could have the life he always wanted in his mind, and this person laying beside him filled one need the vessel to get his relief when he missed and needed Ennis so badly. He could get all the loving, all the hugging, all the snuggling and all the face to face stuff he dreamed and yearned for, all with his eyes closed but that is about as close as he ever would get all that he prayed for.
Ennis loved and wanted to be with Jack the one he loved. The one he could be himself with without judgement and a lot of forthought. He enjoyed the sex, sure enough but it was Jack himself that made Ennis do what he did, change his life, put everything he had in jeopardy just to spend some down time with Jack. He needed to rejuvenate and get his will to carry on with the life he was living. Jack was the only one that could do that for him and without Jack in his life there would be no Ennis. Sure he would have gone through life, but he now knows what happiness is and he needed it.