It's true that Jack's father abused him from early on, when Jack was but a small child. But I think the disdain grew even more as Jack got older and his father realized his son was "queer". I think that was the real reason he did not want the ashes scattered on Brokeback, and the reason he was so cruel to Ennis. He was frowning upon their relationship in that way, showing how much he disapproved. Both Jack and Ennis had very bad relationships with their fathers, but I don't think Jack's father would have crossed a certain line that Ennis's father surely would have. John Twist wouldn't kill his son for being gay, while Ennis's father probably would. If Jack and Ennis had come and lived on his parents' ranch, well John Twist would have known perfectly well what was going on, but wouldn't harm them physically. Ennis's father takes it to another level...remember that Ennis believed that he most likely was Earl's killer...I think Ennis was completely terrified of his father, while Jack had a very strained relationship with his father, but wasn't scared of him. The fear Ennis had of his father was so strong, that if he had been alive, I doubt Ennis would have had a romantic relationship with Jack at all, no matter how much he wanted to, the fear of what his father would do would have not allowed him to.
Back to the topic of Jack "quitting" Ennis...this film has so much of that, characters saying lines but what they mean is something completely different from what they say aloud, the "I wish I could quit you" line is a perfect example of that. Jack loved Ennis and had spent twenty years trying to get him to listen and make the small dream come true...the dream of a couple in love, being together all the time, living together, but over and over again Ennis denied him that, because he could not overcome his fears, Ennis oblivious to how much he was hurting Jack's feelings time and again by doing this. But Jack's love for him was unconditional, and so Jack would never give up on him. In the end, "I wish I could quit you"...it means "I love you, I love you, Ennis, no matter how much hurt it's caused me, no matter how much this living a lie is tearing us apart and denying us our right to be together"...it meant Jack would never give up. Ennis was the one true love of his life and Jack was in it for the long haul. This is why Ennis ultimately suffers the most, Jack dies knowing he did everything humanly possible to be the one he loved, while Ennis lives for years after, lonely and with immense sorry and regret that he was a large part of the reason for their suffering being apart, and with the added sorrow and regret of how much Jack's feelings had been hurt, while Ennis hadn't even realized it till then. Ennis learns the lesson, but pays a heavy price.