To me the answer is in the short story. In the short story both Jack and Ennis are more like movie Ennis, the quiet type. In the movie Jack was more open and friendly. The intimacy part comes into the story through them opening up to each other and becoming friends, before this they were very solitude types.
In the tent Jack was aroused, that much we know, and perhaps Ennis was also, not all arousals are from sexual feelins. Ennis had been drinking and he might have been aroused due to that strain, but when Jack initiated the sexual contact it quickly became a sexual arousal, it does happen.
After reading the short story, which was written very well, I prefer the movie. Being a short story, the plot moved too fast, and that connection was just made too fast. We know Jack had these feelings, but Ennis never did, and then after one night he changes, the next morning there were no doubts, like there was in the movie.