Yes,I tend to agree with you,WcI in that it's not the most important of our boys tragic story;that's for this reason that I quoted Ang's words about the more or less explicitness of their sexual living...After that,the reader's imagination can run free,as it has been in the many,many fanfics that have been written about them.As you say,is easy to suppose how could two 19 years old boys behave,being in love and alone on the mountain with a lot of "crazy" hormones inside them ¡.
However,I'd like to talk a little about Jack's previous experience; it's true that nothing is concreted neither in S.S. nor in the movie...But I think that the fact that Jack takes the initiative in his sexual advances can suggest he's not only more open and,maybe,bolder,but that he has done it before.As a matter of fact,Ang also said that he filmed the scene when Ennis is washing himself while Jack is trying to take a glance at him to show precisely he had a previous experience.That Ennis doesn't know what he has happenned to him,but Jack knows how to manage his tendencies because he knows what are them and how to put them into practice.So,for me,the fact that Ennis is the first in seeing Jack is a simple matter of being in a place or not; and if he avoids looking at him is because he's shier,even if surely he feels a little attracted for this gorgeous cowboy.