I don't think the story is a commentary about modern marriage as much as it is about what happens to Macon after the tragic death of his son and what he discovers about himself in the process of healing. The themes of the story deal with death, apathy,passivity, and isolation from the outside world.
Macon does learn that when he's around Sarah, his wife, he becomes someone who's aloof and disconnected from emotion and any kind of contact with people. He withdraws into himself, not allowing anyone else in. He didn't realize Sarah's effect on him until after his son died and he met Muriel.
Muriel encourages Macon to open up to her and express his grief, beginning Macon's reconnection with the world. He changes from a man who doesn't care about anything (apathy) and doesn't want to become active in anything (passivity) to a man who is strong enough to make his own decisions and face the challenges of life. He had isolated himself from the world, and Muriel helps him to come out of his shell and become an active participant in the world.