Yeah, I forgot about the "once burned" crack. That could have annoyed her. Although "once burned" could also mean something less insulting, such as, "I've already tried marriage once and was hurt when it didn't work out." Which, judging from his expression at the divorce procedings, is true.
Jeff, good point about her being annoyed at the girls still idolizing their father. Alma looks pretty peevish after the rodeo story. But I don't think the pregnancy would make much difference.
Del, I think even Movie Alma had other reasons for divorcing Ennis that may have been higher on the list than his affair, or at least close behind it. The main one: money. (And, of course, the two are connected.)