Tell you what, I think that Jack's trip to Mexico that we see after the Ennis divorce debacle was his first trip to Mexico, but I don't think it was his only trip to Mexico.
As much as I love Jack, as much as I hurt for him after the post-divorce scene, to some extent his going off to Mexico makes me think of a little kid who hasn't gotten his way. In that sense I agree with Momof2's "I'll show him" comment.
However, I've also come to feel that there is more than one layer to Jack's eventual remark to Ennis, at the lake, that he can't make it on a couple of high-altitude fucks once or twice a year. I think Jack means this literally--he has a stronger need for male-male sex than Ennis, who is able to wait for his meetings with Jack--and thus I think Jack's trips to Mexico continued over the years, after the one we see following Ennis's divorce. But I also think the deeper meaning of the remark is that Jack needs the emotional intimacy, and he's not getting this from one or two fishing trips a year with Ennis. I think is what is behind Jack's comment the night before the confrontation that sometimes he misses Ennis so bad he can hardly stand it.