This is one of the best questions of them all. And I don't know if there's another thread about this, but if there is I hope the moderators will move this discussion to merge with it.
This line is obviously open to many interpretations. For me, it suggested the following: The open space referred to a difference or cognitve dissonance. What he knew was that he had overpoweringly strong feelings for Jack, and vice versa(love, really), and what he tried to believe was that neither of them were "queer". I think that even after all that the two of them had been through, together and separately, during the course of their relationship, Ennis would still have balked at the idea of applying the label of "queer" or "gay" or "homosexual" to either himself or Jack, but especially to himself. This is in keeping with Annie Proulx's statement that the primary theme of the story was the destructive effects of rural homophobia, both internal and external. Like many others, he might have been able to accept the term "bi-sexual", but the problem with that is, in the story, he had already told Jack that sex with women couldn't compare to the sex he and Jack shared, and I don't think this was just an issue of "mechanics". He was, in fact, sexually functional with both men and women, but his stated preference was for men, or in this case, man.
And an interesting thought: if he had passionately loved and been attracted to just one woman, I think there would be no argument with the notion that he was, by nature, "straight", regardless of whether circumstances, fears, or social pressures (in a very different society than the one in which we live) led him to have sex with others, whatever their gender.
samc,
Dang that Annie Proulx and her "loose ends"!!! Look at what she's done to me! (and a few others as well...)LOL