Here we go, I just joined this forum and here's my fist post!!
I think they for sure have sex in other positions than the good ol' doggy, and I have several pieces of evidence to support my claim.
In the movie, the second time they have sex - the first time Ennis comes to Jack - Jack holds him to his chest, then rolls on top of him as they make out. My idea is that Ennis sort of want Jack to lead, and while I don't think that Ennis ever was on the receiving end, I do think that he enjoyed letting up his "control" to Jack sometimes.
My absolute number one favourite line from the shortstory supports this theory, as well -
"You do this with other guys, Jack?"
"Shit no," said Jack, who had been riding more than just bulls [...].
This doesn't just
imply, but straight out tells us that Jack rides c*ck, which means he'd be on top. [But he is a cowboy after all, 'course he'd love that position

] Sure, this doesn't prove that Jack rides Ennis' c*ck, but in combination with aforementioned scene from the movie, I would say it's not too unthinkable.
That's where my third evidence comes in, as they're in the Motel Siesta, it says they're
jouncing a bed, and the dictionary definition of jouncing is
to jolt; to shake, especially by rough riding or by driving over obstructions, which suuuure, it could mean Ennis is just going rough, but in combination with the description of him "laying spread-eagled, spent and wet, breathing hard, still half tumescent," definitely sounds to me that it indicates that Jack had been riding him.
Though, as I am both the prosecutor and the attourney I will admit that Jack's following words does not, however, support that claim, for he says "Christ, it got to be all that time a yours a-horseback makes it so goddam good," which could
first of all be read as Ennis was riding Jack, but I highly do doubt that Ennis ever did anything but give, so the other alternatives which are more likely would be that Jack is referring to Ennis' stamina and/or his pelvis work.
But, coming to my own defence again, this could still mean that they face each other. I present more proof to support this claim, which might also be the strongest piece of evidence in my eyes. It's from their last trip, and [scaling it down to only what we are looking for here] it describes their sex like this.
Ennis put his arm around Jack, pulled him close [...] Jack slid his cold hand between Ennis’ legs [...] said Ennis, undoing buttons [...] Without getting up he [Jack] threw deadwood on the fire [...] and they rolled down into the dirt.
This, to me, seems like they are either 1. sitting next to each other, 2. facing each other while sitting down, 3. Ennis pulls Jack on top of himself. It doesn't say whos buttons Ennis is undoing, but playing with the thought it's Jack, they
have to be facing each other. Then they roll down in the dirt, and that to me confirms that they are indeed facing each other, most likely entwined in each others arms and they "couple" like that.
And my thought on Ennis taking Alma from behind I think it's because he can imagine it's Jack he's !@#$, not necessarily because that was how they always did it, but because he doesn't have to see Alma's face and body. I wonder if he ever had to stop himself from moaning Jack's name. He does confess that he jerks off to memories of Jack "a hunderd times".
Last piece of evidence I have is the fact that they kiss. Had it been specifically stated that they never kissed I would've found it more plausible that they also never had sex face-to-face because then it would be more obvious that Ennis did not want to see or feel that it was a man he was doing it with, but considering that they kiss I never got the feeling that he doesn't want to acknowledge the fact that he is with another man.
But there's also a pretty big difference between book-Ennis and movie-Ennis, because in the books Ennis is way more verbal and
does actually talk about his feelings, his fears and his love for Jack, whereas in the movie he's more closed off to expressing himself. I'm thinking specifically about their reunion there are quite a lot of quotes from Ennis where he talks about his feelings.
"Four years. I about give up on you."
"I like doin it with women, but Jesus H., ain't nothing like this."
"When we split up after we got paid out I had gut cramps so bad I pulled over and tried to puke, [...] Took me about a year to figure out it was that I shouldn’t a let you out a my sights."
"There’s no reins on this one. It scares the piss out a me."
"We can’t. I’m stuck with what I got, caught in my own loop. Can’t get out of it."
“I goddam hate it that you’re goin a drive away in the mornin and I’m goin back to work. But if you can’t fix it you got a stand it.”
Whereas in the movie, I think the closest think we get to a confession of love we get is the movie-only line "It's because of you I'm like this, Jack. I'm nothing, I'm nowhere. [...] I just can't stand this anymore, Jack." and of course "If you can't fix it, you just got a stand it." though that's also a bit ambigous in general.
Either way! By now I'm just rambling nonsense, I've made my point. I think they for sure had sex face-to-face, because they loved each other.