I think all kinds of relationships can exist and each one of them is according to each couple circumstances; because human nature is complex,even if narrow.minded persons insist in telling us that everything must happen in a clear and concrete way always.That's not true; life is not a straight line, but an ellipse.You don't know when and where it begins nor when and where it'll finish.
As Ennis and Jack regards,I also believe that they were already in love when FNIT happenned,even if hidden under a cover of sexual desire-and even if,of course,none of them knew for sure what was its name-.But it's also true,IMO,that this night means the physical concretion of this desire,something that was forging between them for many weeks and that the concrete circumstances of it made "explode".For this reason,I said in my former post that SNIT was the consolidation,and half acceptance,of love,not only sexual desire.