I don't know if there is a thread about when Ennis and Jack first realized they were in love, but I think it was at the time of the reunion embrace. I think, up to that time, they missed each other, but Ennis probably didn't expect to ever see Jack again, and Jack wasn't sure Ennis ever wanted to see him again, so their love remained new and untried up to that point. At the reunion, though, everything broke loose. They were out of control -- almost lost it completely, as you mentnioned above. Get me to the motel, quick! I think that's when they both realized that what they had was something that would last. It had withstood a four-year separation; what else might it survive? (As we know, now, it even survived death!)