Marriage is more than just a legal agreement. It is central to our way of life. Look at how kids react to divorce. Even when they know they will still see both parents, the idea that their parents do not love each other any more is devastating to a child. Without realizing it, kids automatically buy into the idea that two people should commit to each other on a permanent basis. When they see their parents fail to follow through with this commitment, it destroys at least some aspect of their belief in the "rightness" and viability of committed love. It's a feeling that is similar to losing a loved one--you know that something has been lost that can never be recovered. But kids would never be able to put it in those words.
I'm not saying divorce by parents is never justified, just that it is almost always a shattering experience for children.