I see what you are saying about the "decides." Setting that aside just for a moment, what it appears to me that he's doing when he's sitting by the fire is gathering up his courage. He's nervous but excited, maybe thinking something along the lines of, "OK, here we go. Am I ready for this? I hope so." Meanwhile Jack, inside the tent, looks like he anticipates Ennis coming in and is getting undressed in preparation. When Ennis does go in, they both look like that's exactly what they expected to happen.
So what about the "decides"? I guess from this perspective it means either that he decides that OK, here I go, now's the time, or maybe that he is still harboring some tiny bit of doubt and decides to ignore that and go for it. Or maybe it's just one of those stage directions that fell by the wayside, like the one about Ennis sending Carl a murderous look.
In any case, even if the "decides" just doesn't make sense under this interpretation, to me it makes no less sense than Ennis' open-ended verbs in "this is a one shot thing we got goin on" under the other interpretation. That is, either way you look at it, you have to ignore something incongruous. But if I have to ignore something I guess I'd rather have it be in the screenplay than the movie itself.