Agreed, Star and Del.
When Alma does kick Ennis to the curb, in Annie Proulx's story economics seems to play a considerable role in the decision, in my reading, with Ennis's relationship with Jack only one factor. Before that, her opportunities were pretty limited--a part-time job in a grocery store, with two little ones to care for. We don't know whether she even finished high school or was a drop-out like Ennis.
I've never really given this much thought until now, but I'm guessing Monroe (or Bill, in the story) was waiting in the wings for the divorce to be finalized. When Alma married "the Riverton grocer," she was clearly "marrying up" the economic ladder. The film illustrates this pretty well in the Thanksgiving dinner sequence.