Ok. I have read all the posts in this thread and some of them several times. It appears to me that most of you believe that when Ennis tells Jack he hasn't yet had the opportunity to be a sinner he is making a pass a Jack, or at the very least letting Jack know he would be interested.
I couldn't disagree more. If Ennis was interested in having sex with Jack, then why did he jump up ready to fight Jack on the first night in the tent? It was very obvious that Ennis was surprised to find his hand where it was and he did not like it. Even if Ennis wanted to have sex with Jack but did not want to be in the passive position he would have told Jack so and probably would have told him to drop his drawers.
No, Ennis was shocked and angered to find them in a sexual situation and jumped to his knees to fight Jack. But he couldn't hit his best, and only, male friend. And Jack calmed him down. Then Jack, once he knows Ennis will not hit him, takes off his coat and unbuckles his pants. Ennis, pretty well liquored up, says, ok you little faggot, you want it, I'll give it to you, and attacks Jack in a very rough, if not violent way. Ennis' sexual urge was just too great to resist the offer. The next morning he was ashamed of what he had done, and maybe a little ashamed of the way he had done it to his best friend.
So no, in my opinion all Ennis was doing when he said he wasn't a sinner was admitting to his best friend that he was a virgin. And when he looked down and away after saying it, it was out of some slight embarrassment. He was, after all, a nineteen year old male.
This scene is yet another in a series of scenes to show a growing bonding between the two boys. They talk about each others religious upbringing with Jack being a Pentecostal and Ennis being a Methodist and Ennis admits something no nineteen year old boy readily admits to another nineteen year old boy, that he is a virgin. A very deep bonding took place at that moment. Ennis showed he trusted Jack enough not to make fun of him for being a virgin. He wasn't suggesting to Jack he wanted to get it on with him.
And that is my opinion.