Tension and conflict in a marriage inevitably lead to a phenomenon called triangulation. What that means is that a third person, usually a child, is unconsciously drawn into the parents' conflict as a means of diffusing it. In other words, a wife who feels angry with her distant husband might compensate by becoming closer to her son. The son, who also feels his father's distance, may move toward his mother in response to her overtures to him. However, the son feels uncomfortable in his role of husband substitute and eventually acts out his discomfort, usually self-destructively.
This is exactly the dynamic that is occurring between David, Marcia and Andrew. It seems that only Andrew has a clue about the role that he has been unconsciously conscripted to fill. He tells his mother, "Don't drag me into your problems.... If you want to leave him, do it for your own reasons." Earlier he tells his mother, "Quit defending me, Ma. I can talk for myself," but Marcia doesn't hear, she continues to act as if Andrew needs and wants her to be his protector.
It's not uncommon for a parent or parents to drag the reluctant child right back into the middle of the relationship. It's as if the parent is begging the child, "Please act out so that Dad and I can deal with you. It's too hard to deal with our marriage." It's a remarkable child who can look at the parents and say, "Fight your own battles. I'm out of here."