It isn't bad if you are in an objectively bad marriage ofc. But if you're in a marriage that "can be worked out" then yeah it's a bad thing, for the kids especially. There is definitely something to be said about making things work because there is no easy "out" called divorce.
In a healthy society, divorce should be difficult, expensive, and rare. Social incentives should be structured to make it less appealing than working on the marriage. Conversely, marriage and children within marriage should be incentivized by law and social programs. It may such policies are unworkable in our present configuration.
