Being with Alma and Cassie was not "normal" for Ennis, it never was. Ennis as he is with Jack, is the most normal he'd ever been in his life, and could ever hope to be. Monogamy, yes, why shouldn't he expected that from Jack. Understanding that this is not a realistic idea, given Jack's nature, and compounded by the suffocating rules set down by Ennis, faithfulness was indeed expected.
Jack was his, and Jack will always be his. What Jack couldn't understand was Ennis resistance. If Jack would have understood it and dealt with Ennis's fears head on, he may have help Ennis to understand that it would be alright, just trust him Ennis and Jack were not introspective men, the complexities of their relationship was just too much to take in, the here and now was all they could see.
Ennis with ease could discuss their experience with the women in their lives, but to discuss other men, was a territory unimaginable, an undiscovered country so to speak. Why this envelop wasn't pushed a little more in this story and not just limiting it to 'Mexico', well that's why we're discussing the what ifs now.