I think you're right Ladyeve that Ennis was admitting to Jack that he was a virgin. All during the movie, even when they had known each other for nearly 20 years, they spoke in code or body language, because that is "the cowboy way." Works great sometimes, but when they misinterpreted each other's code, those were the heartbreaking moments!
Just a clarification, when Ennis spoke those words about "you may be a sinner" they had together drunk about a half-bottle of whiskey (it could have been Wild Turkey, but I think it was not that good quality, definitely bourbon whicsky tho). Jack handed Ennis the bottle with an interested "mmmm!" sound, and then Ennis killed the bottle, which explains why he was staggering into the tent, while Jack was keeping his wits about him. The "you may be a sinner" line was very interesting because the way Ennis said "sinner" it did not have the connotations that it usually has. It was like Ennis looked up to Jack because he was a sinner, that he had "taken the opportunity."
Squashcourt, your ideas about Ennis are right-on. If you look at the story, there are several phrases that show that Ennis looked up to Jack, such as "he was grateful for the companionship, where none had been expected."
Keep the questions and ideas coming!!