I wonder whether jack ever told that much to Ennis -- so technically, he was not lying if he never told him anything.

Perhaps we should also consider the things Jack intentionally hid from Ennis.
Ennis's knowledge of the Mexico trips is quite ambiguous. Did Jack tell him about such trips before, or did Ennis just deduce Jack's trips from the comment in the last meeting? I would think the former is more likely.
I think that most of these things Jack kept hidden were of a transient sexual nature -- nothing like Jack actually getting deeply/romantically involved with another man. Even the affair with Randall was probably more for sexual gratification in the beginning (at least up until the last meeting).
Ennis is Jack's one great love. All else was of a purely physical need with no depth. When all hope for a shared life with Ennis was lost in the last meeting, it was only then that he probably considered someone else -- but death held that love pristine, mercifully. And perhaps Jack was only 'thinking out loud'. I canot imagine him ever forgetting Ennis, so long as he and Ennis lived.