A couple is unhappy but they stay together for the sake of the children. This is so common. I did it myself, despite the abuse. I thought it would be worse to break the family.
But nobody is perfect. Even with the best intentions, sometimes one parent can be very detrimental to our development as a healthy person.
What if having a step-mother or step-father could help a child get another view on life, another perspective? What if it would allow the child to experience what love and respect feel like when two adults are honest with each other? Instead of pretending that all is well when they are forcing themselves to stay?
Nothing is easier than the truth in the long run.