IMO, the scene is too abrupt and knowing the personality of Ennis and how he would have reacted if Jack had suddenly forced Ennis to touch his penis, I just believe that Ennis Del Mar would have given Jack Twist an attitude adjustment first, by slugging him, and then ask questions later.
Anyone who would say "I'm not no queer" and literally believed that to be true about himself and be an Ennis type, too, would not have let another man even touch him if sex was going to be involved.
Well, the scene worked brilliantly for me. I believe Ennis wanted Jack; he immensely enjoyed Jack’s company, was secretly attracted to him (perhaps on a sub-conscious level?), and lusted for him. I think a lot of the subtleties preceding the consummation of their relationship build up to the scene.
See I think Ennis
wanted to believe he was straight but on the inside, he knew. He just could not come to terms with his sexual orientation.