You're right; there must be an element in Monroe of wanting to show Ennis all the material things he can give Alma, and that she feels free with him to have the child she couldn;t afford while with Ennis. Maybe Alma feels something like the same - she is in a way paying Ennis back for all the years of uncertainty, all the times he went off with Jack, all the lies, expressed or implied, that he told her, the poverty, at least part of which was attributable to his taking only poorly-paid jobs he could dump at a few weeks' notice. Now she is secure and relatively prosperous, and he's on the outside looking in, as she once was when she saw him with Jack.