Late to this discussion but in my view, Cassie was a disguise. I saw nothing about her that was anything like Jack. The only similarity was that she, like Jack, reached out and pierced Ennis' shield. But otherwise, Cassie was FEMALE and Jack was MALE. Only if Ennis was "bi" could Cassie have a shot at meaning to Ennis anything like what Jack did. And that Ennis could simply walk away from Cassie, that he could simply ignore her and go back in his shield tells me that Cassie was simply a diversion, a last ditch effort at being "normal" and Jack's last speech to him finally showed him for what he was: queer. So if Ennis hadn't met Jack, would he have been happy with Cassie? No. Because he'd still be in a crap marriage with Alma. Alma only drew away from Ennis and the marriage after seeing her husband making out with another man and quitting jobs to spend his only vacation times with that man (and not her and their kids). So no Jack, no failed marriage, no Cassie. On a side note, Cassie was one sentence in the short story...further highlighting her lack of importance to Ennis or the story.
It just occurred to me, that it was not so much Jack and Cassie pierced Ennis' shield, because although Ennis built a castle to protect himself against emotional commitment, he did marry Alma and had relationships.
I think the metaphor to use here, in my opinion, is that Jack and Cassie are two pitchers filled with love, with affection to spare, and saw Ennis as an empty vessel that was willing to receive their affections.
No one else bothered with Ennis, saw through to his soul and understood his desires, his needs. Jack and Cassie saw, understood, and poured their heart into Ennis.
Cassie had no chance with Ennis because he was already filled with Jack's affections, and had no room for others, even though the few times a year meeting with Jack were not enough to quench his thirst for love.
Ennis and Jack were two souls, two vessels, that fitted one another that one summer on Brokeback Mountain. The rest of the movie was just about the subsequent intrusion of society into their relationship, starting with the peeping and cutting Aguierre, then the mocking minister at the wedding, and the hardship Ennis faced trying to provide for his family.
In the ideal world, when two souls meet, and one of them has the love to give to the other, and the receiving soul is made better, is healed from the love, they merge as one. Jack, in many ways, was also healed by loving Ennis, since he was always told that he was no good, except perhaps by his ma. To be needed, and to be appreciated, was to Jack, the same as to be loved.
In that sense, I think Ennis, if he never met Jack, and still somehow divorced Alma, might end up marrying Cassie in that alternate universe, simply because Cassie was good enough to fill his emptiness. They may not be the soul mates that Jack and Ennis would make, but they would be happy nonetheless.
Just my two cents.