I don't think Ennis had the internal vocabulary to explain to himself just what it was he was feeling. If you give something a name, it becomes more real. He simply called it a one-shot thing and was thinking about the fact that he was already committed to marrying Alma after they finished their job.
I do think that his gazing at Jack, especially when the latter was riding the horse up on the mountain as the sheep grazed and Ennis was in the stream, was part of his unnameable feelings.
Jack, on the other hand, seemed smitten with Ennis from the time he pulled up outside Aguirre's trailer in his truck.