If you read Annie's story again, you will note that Ennis "enters" Jack with the help of "slick" and a little spit. Male anatomy being what it is, that requires Ennis to have an erection and the benefit of a little time, a minute or so. The implication being that Ennis was lying there wanting it pretty badly too, he just wasn't as consciously aware of it yet. This may be just as true in the film.
It would be interesting to time how long an interval it is between Ennis bolting upright and then undoing his belt and trousers; can't be more that 10 seconds.
And maybe OT and a little off color, but I can't help feeling a little distressed for our poor Jack on the suddenness of Ennis's entry. OWWWW!!! to put it mildly. Good thing Jack doesn't have a low startle point. OK I promise I'll stop now
Oh, I don't think your 2nd paragraph is OT nor OC in that some people who are bottoms do need considerable foreplay before they are entered from behind.
Ennis did have to have some sexual arousal and was sexually attracted to Jack. I have met bisexuals who don't have a problem with foreplay with women but, they are in a hurry when it comes to sex with another guy.
In the movie, from the way that I remember it, I've only seen it 3 times, it was almost like they were not even touching before Jack reached for Ennis's hand. And, I doubt that a person who had been dizzy drunk like Ennis was would even get an erection that fast as in the movie.
I prefer Annie Proulx's "deepened intimacy" in this situation. As to whether they kissed or not, she did not say; but, considering how they kissed at the reunion 4 years later, I would not be surprised if that intimacy did include some kissing. If Jack had been lying on his left side facing Ennis it would have been not problem for Jack to take Ennis's left hand and put it on his erect penis.