I would also say that the moral of the story is the consequences of stubbornness, reminiscent of that expression about cutting off your nose to spite your face. Note that the man and woman's refusal to bar the door results in two robbers coming in, eating all the couple's food, threatening them, possibly, with bodily harm and sexual assault. When these threats surface, the man is finally angered enough to speak, too late, of course. But all the wife can think of is her victory over her husband, rather than the harm being done to both of them.
In terms of marital counseling and advice, this is a powerful story, symbolic of the foolish stubbornness that couples sometimes indulge in, harming a marriage, harming children, harming material assets, all in the name of a victory over the other. I would counsel a married couple who was engaging in this behavior to remember to remind one another to "Get up and bar the door" as a humorous and gentle warning about how such behavior can end.
The theme is stubbornness. Both the man and his wife in the ballad are stubborn. They are tired everyday and each one does not want to do anymore than what is necessary. They make a rule: the person who speaks first must bar the door. They stick to that rule because each one is committed to winning out over the other. And they will not move an inch to help the other. In the end, a man threatens to take away the wife and he speaks up, of course, breaking his vow of silence. Therefore he must bar the door.
Your advice letter might address stubbornness. It might address the issue of getting along between two people who live together.