Jeff, I think that you are correct! I hadn't considered that Ennis was entirely aware of Jack's plans, but now I think he knew what was going to happen.
AP writes, "...with the help of the clear slick and a little spit, he entered him, nothing he'd done before but no instruction manual needed."
The "clear slick" part of that qoute tells me that Ennis may have been erect for a while. Maybe Ennis was playing that old game my frat brothers used to play, he was pretending to not be interested, just laying there waiting. "oh last night, I was asleep, sure was a lot of noise in the halls!" yep, I've heard that one before.
Good thought! (Wrangler slaps self up side the head again.

)
Can you believe I'd forgotten about "the clear slick" part? But if Ennis was already "leaky," that would just about require that he was already erect, wouldin't it?
Not sure I'm convinced that Ennis knew what was, er, coming. I'll have to think about that some more. The scenario in my own mind at this point is that neither was actually planning it or even expecting it that night, but once they got snuggled up together, they both got hard. Jack could feel Ennis against him, then reached around, took Ennis's hand, and placed it on his own erection. That action gave Ennis the "go ahead."
I wonder, however, if this scenario quite works in the film? Maybe Heath's drunkenly mumbled, "What're you doin'?" suggests that "Film Ennis" wasn't quite so ready? Except that "it happens" pretty quick in the movie, too. ...

P.S. Wrangler, it did seem instantaneous and I thank you for explaining why! Sandy slaps forehead!
Well, I just offer that as a possible explanation, but, you're welcome!
