"Too late to go out to them damn sheep," said Ennis, dizzy drunk on all fours one cold hour when the moon had notched past two. The meadow stones glowed white-green and a flinty wind worked over the meadow, scraped the fire low, then ruffled it into yellow silk sashes. "Got you a extra blanket I'll roll up out here and grab forty winks, ride out at first light."
"Freeze your ass off when that fire dies down. Better off sleepin in the tent."
"Doubt I'll feel nothin." But he staggered under canvas, pulled his boots off, snored on the ground cloth for a while, woke Jack with the clacking of his jaw.
"Jesus Christ, quit hammerin and get over here. Bedroll's big enough," said Jack in an irritable sleep-clogged voice. It was big enough, warm enough, and in a little while they deepened their intimacy considerably. Ennis ran full-throttle on all roads whether fence mending or money spending, and he wanted none of it when Jack seized his left hand and brought it to his erect cock. Ennis jerked his hand away as though he'd touched fire, got to his knees, unbuckled his belt, shoved his pants down, hauled Jack onto all fours and, with the help of the clear slick and a little spit, entered him, nothing he'd done before but no instruction manual needed. They went at it in silence except for a few sharp intakes of breath and Jack's choked "gun's goin off," then out, down, and asleep. "
From reading it again, I'm getting the sense that it was more mutually started than in the film. Ennis didn't start out sleeping outside but rather went into the tent right off the bat. They had gotten intimate even before Jack put Ennis's hand there.