This is a very interesting question to me. I am a gay man myself, and when I first saw Brokeback, back on February 18th, I felt that Ennis was a primarily heterosexual man who somehow formed this extraordinary bond of love with another person who happened to be a man. An important component of this interpretation was the fact that nowhere in the film do we ever have the sense that Ennis is attracted to any other man than Jack. From my experience and observation, this is very unusual for a homosexual man, though it is not impossible.
For the record, my sister, a straight woman, also had this same feeling that Ennis was an otherwise straight person who happened to fall in love with another man. She felt this was one of the most provocative and radical implications of the story, that the heart can lead us in directions that we might never have imagined going.
Over time, I have now come to interpret Ennis as a very repressed homosexual man. There are a number of valuable albeit sometimes subtle clues to support this view. There is a lot of evidence that Ennis is attracted to Jack from very early in their association, quite possibly from the moment he first set eyes on him. By the time of the first tent scene, there is no question that Ennis is attracted to Jack physically as well as emotionally, regardless of the specifics of his sexual identity. But later remarks that Ennis makes in the story, including the "Do you ever think people know?" comment to the "You're the reason I'm this way" rebuke in the lakeside quarrel, strongly suggest that, at least by the time he utters these words, Ennis has come to see himself as homosexual (or at least as a man attracted to another man), however much he may hate to think of himself this way. By story's end, Ennis can certainly no longer doubt Jack's homosexuality, and I think he has essentially come to terms with the realization that theirs was a homosexual relationship, if not necessarily with the conception of himself as specifically homosexual. He certainly loves Jack, a gay man, is conscious of this love, and this suggests to me that he may have accepted a homosexual identity for himself.