So true. I think Ennis tried to fend it off, but his sexual desires took over, similar to the Reunion kiss. To me, it shows Ennis was so turned on and attracted to Jack prior to FNIT. My husband, who FINALLY watched BBM with me the other night questioned why Ennis just didn't go in the tent to sleep when he first decided to stay. I gave him the "duh" look (God bless him, it took him a while to build up to seeing the movie, so I couldn't be too harsh). Because Ennis was hot for Jack and didn't want to get too close for fear of letting go of his sexual desires---at least that's what I thought!
I agree. The little looks Ennis gives Jack leading up to FNIT, when Jack's showing off on the horse, on the side of the mountain etc, Ennis seems SO captivated by him - maybe because he's a lot of things Ennis isn't, more open, extroverted, and yet genuinely seems to want to get to know Ennis and get him to open up, it must have built him up, suddenly having this interest shown in him. The fires were definitely well stoked by the time FNIT came around, Ennis just needed that come on from Jack, and when he got it - no holding back, and good news for us - cos that scene is HOT! 
The irony, for me, is that Ennis probably never thought in a million years that Jack was just as hot for him as he was for Jack.Of course, we all know the score, but I don't think Ennis could fully understand himself what he was feeling for Jack, so it would never occur to him that Jack felt the same. Of course, once Jack made it perfectly clear that they wanted the same thing, there was no stopping Ennis

Like the ss says "nothing he'd done before, but no instruction manual needed", Ennis just literally did what came naturally to him