I wonder if Jack ever told Ennis what his father has done to him.
John was so cruel, violent and humiliating towards Jack.

I mean, in the SS he hit him with a belt and peed on him. Jack was only a child. This is a trauma, a big one. And it lived inside Jack's heart, even when he was with Ennis.
He was traumatized: in fact, he was not scared by pain...rodeo, sex as bottom, are fitting exampes: some may say he was brave, I say he did not scare easily because he grew up dealing with a deep fear of that monster father he had.
I hate so much this happened to him.
Maybe, in some way, being loved by Ennis also gave him back some fatherly bond he had never experienced. IMO, Ennis always seems more mature than him.
I think Ennis did not know Jack was a victim of abuse. I think Jack was too ashamed to really speak about it with him. Ennis may have interpreted it as weakness. Jack did not want to appear weak in Ennis' eyes.
However, I wonder how would Ennis react if he knew?
Would he have behaved differently towards Jack, if he knew?
I don't mean he had to feel pity for him or be compassionate, but, maybe, understanding that Jack was abused as a child may have helped Ennis understanding Jack's deep need for love and his requests for being together.