I'd say that the parents are uneducated on what love and commitment really is.
Sometimes people doesn't get married because of "love" or even "commitment". Sometimes they get married because of familial or peer pressure. Sometimes they get married because they got someone pregnant. Sometimes they get married because they settle for someone 'cause they don't want to be alone. Sometimes they just make a bad decision.
Even if those reasons are the wrong reason to get married, people still get married for those reasons, and when they do chances are either or both members of said marriage will develop resentment towards each other (which the kids will pick up on), and possibly towards the kids (which they subconsciously see them as the chain holding them to someone who makes them unhappy). It can be exacerbated further if one parent is a step-parent that doesn't have the "blood connection".
There comes a point in which "powering through for the kids' sake" does more damage than good. The kids pick up on the dynamic of the failed/failing marriage and how the couple interacts, and it stunts their perception of a healthy relationship since the parents are doing everything in their power to create the illusion of a healthy relationship when it's rotten to the core. It can also stunt the kids perception of their role/place in a healthy relationship, or the place of their partner in the relationship.
I.E. If the wife is verbally abusive towards the husband, and the husband is trying to be a "gentleman" in the relationship and doesn't confront her. Both decide they want to stay together "for the kids sake", but neither one of them are willing or able to alter their behavior, it can damage the kids: A son would see how his mother behaves towards his father as "normal" and with the lack of resistance to her behavior coming from his father, would think his role is to be 100% submissive and not stand up for himself. It would stunt his views of a relationship being a "good" thing and stunt his view of women and men in general. On the flipside, the daughter would see how her mother treats her father as "normal" and would think this is the appropriate way to treat a man, and since her father is doing nothing to dissuade this, it would appear to her that men are supposed to be dominated and and thus stunt her view of the dynamic of how a healthy relationship should work, which would affect how she sees men and women in general--not to mention affect all of her future relationships.
It does have to be said that every gender there can be reversed and the same applies.
Now, if the husband and wife split up, it would do a less damage to the kids since the child who shares the gender of the abuser would see that they can push someone too far and they can stand up for themselves and up and leave. The child that shares the gender of the abusee would see that it is okay to up and leave and that you don't have to be a doormat.
Anyhow, enough on this since it's taking away from the congrats Urblock deserves for 36 years.