Here's a thought that just occurred to me that you might want to kick around: On Brokeback, Jack and Ennis have agreed to agree that they are 2 straight boys engaging in some sport sex for the summer. That's what I think "I ain't queer, me neither," means. - We can screw our brains out all summer long, as long as we don't mean it. We've agreed that neither of us is queer so, in the long tradition of lonely, separated from the rest of the world heterosexual male pairs, we can have as much sport sex out here as we want, and no one but us will know, so we won't have to pay for it back home.
But one thing happens on Brokeback that we don't find out until the big fight, when Jack remembers the drowsy embrace. It stands apart because it is "artless and sexless." With no thought, plan or ulterior goal, Ennis comes to him at the fire to say goodbye for the day, and he is in no hurry to let go and get going. I think he remembers it at this time because that's the first time he thinks that maybe there's some "feeling" behind the sex, after all he's really liking this and this ain't sex. It might occur to him to wish that maybe they aren't two straight guys after all. He's warmed further at the thought, but knows it can't be because Ennis is straight - they've talked about it and have graciously accepted each other's lie for truth. Of course, they're both so infatuated with the idea of each other that neither of them knows or could even think that the other guy is lying, too. But this time their contact is different: Ennis holds Jack and rocks him, hums to him, breathes with him, gives him a heartbeat to feel, and tells Jack to hit the hay as he is leaving. This truly sweet moment is the first point of awareness for either of them that this summer fling but could become a lifetime craving. Ennis doesn't notice any of this, not until after the final fight. That's my vote for when their love did start. Until a new thought comes along.
(Of course they started to fall in love when they first met, but with no experience, they didn't know what they were up to, or what was up with them, until Jack caught on in the DE. Poor bastard couldn't tell "straight" Ennis that he had feelings for him too - might be taken the wrong way.)