Getting back on topic...
I always thought it interesting that Jack and Ennis can easily talk about women: in fact, it's Jack who starts it when he says "all this time you ain't found no one else to marry"--->Ennis mentions Cassie --->Jack lies about the ranch foreman's wife. There's no obvious jealousy at all.
To me, that's their "pact": "We ain't queer."
When Jack reveals "the truth is...sometimes I miss you so much, I can hardly stand it", that statement is awfully close to "I love you", and therefore unacceptable to Ennis. Ennis sort of cowers in response.
I always wondered if Ennis felt too threatened by this, and "made up" the story about August.
Jack certainly looked dejected that next morning. Is he expecting yet another rejection from Ennis?
Then, the whole Mexico thing, confirming Jack is queer. Which means Ennis is queer. Their pact is broken. I think it's less out of jealousy or cheating, but about Ennis being confronted with who he really is. Ennis makes his threat, then the pathetic "it's cuz of you I'm like this".
Ending with the "can't stand this anymore, Jack". Coming from the man who originally said "if you can't fix it, you gotta stand it".
Can't fix it. Can't stand it.