tpe and aintfoolin,
You're talking about what I've thought about myself. There's a set of arbitrary signs that would lead us to think that the events of FNIT was already put into motion. As they emotionally and mentally played that dance of seduction, they physically made moves to each, albeit slight, but can be seen.
Remember, when they first arrive with the sheep how far apart they were sitting, then as time went on the more comfortable they were with each other, the more they knew about each other, the nearer they physically got to each other.
The Elk shooting scene locked it in. That was the moment of trust, admiration, and attraction that lead to the FNIT, (I know I might on own on this), because when they were eating the elk meat, they were sitting right next to each other, a bond was formed.
From then on they couldn't stay away too long from each other, or should I say Ennis sought out Jack from that moment on. On that night of consummation of their relationship, too late to go out to them damn sheep, Ennis stay in camp, too damn cold when the fire died out he didn't relight the fire but made it known he was suffering, and Jack calls him in. "Ennis!" "HUH?"
Now whether Ennis was truly shocked by Jack's move on him, he recovered enough to know what Jack wanted and what he wanted. It was the last step to that dance towards each other that had to be completed.