I won’t say my interpretation is the correct one, but I always saw it as Ennis admitting that he was a virgin. There’s a million little bits and pieces to the scene for me, like Jack probably implying that they were sinners because of their sexuality (or at least regarding himself), and that maybe Ennis’ response to his comment made him enough confidence to act upon his feelings during TS1. All in all, I saw that line as Ennis saying he was a virgin as they both came from religious upbringings and premarital sex would have been a sin. Especially when we see that Alma got pregnant almost immediately after being married (if they were messing around before that, she probably would have gotten pregnant before they were married). Thats how I always took the line, but I’m sure there’s other ways of reading it, too!