I have always thought that,somehow,the s.s. explains the passage from friendship to sex in a more detached and even logical way.Maybe all it's due to the cinematographic rhythm that necessarily must reduce a history's time with its feelings and the way to develop them.In the s.s. it's clear that they were creating an intimacy in the psychological sense based in all what they shared,the way they respected each other's opinions,how they could open their hearts to the other in their most inner aspects,and so on.Being as it's BBM a work based on what is felt more than what is said or seen,full with details where body language plays a great role,I understand that this passage can be inadvertent or at least, not as clear as in the novel.
I think this is the reason why the time passes so slowly during their good times on the mountain,in comparison with how fast it passes the following years;so that we can be able to appreciate the progressive attraction of these two characters.First,and almost without realising it (and,in Ennis case,without admitting it),as men; but,little by little,as a coworkers,friends and companions,by finishing again as men with all the consequences.IMO,also because we realise how short is happiness time-BBM times-and how long is time we pass without the ones we love-the long weeks and,even,months,without meeting each other-.
As regards to Jack's previous experience,I think this first part of the movie also suggests that he has it,for the subtle but continuous task of seducing Ennis that he carries and the lead role that he takes when this seduction must have an active side; for me,it's more than the simple "explosion" of desire in two lonely young boys whose isolated environment can perfectly lead them to that.I think Jack knew what he had to do,when and how to do it,with the "little" difference before Ennis that he had had only sex up to that moment,but then he had love,meanwhile Ennis hadn't had none of them before.JMHO.