You have to remember too, that when Jack and Ennis parted, Ennis was about to get married to Alma, so Jack had to be careful that he was not going to intrude on what may have been a comfortable life for Ennis......after four years, he could just visit him like "an old fishin' buddy" with no strings attached....
Even getting the reply postcard with "you bet", I still doubt, that Jack would have thought Ennis would attack him like he did, with that kiss.....I think he was telling it how it really was, when he said in the motel that he didnt think they would get into "it" again, although he was hoping like hell it might.
When you see the look on Jacks face, when Ennis holds him and kisses him, he is surprised, but then returns the same kind of kiss, but I doubt whether he would have intiitiated it, if Ennis hadn't.