When parents stay together, even though they have an unhealthy relationship, children will be negatively affected. Children do not start drinking, smoking, and using drugs when their parents divorce. They usually do these things before. They also need psychological help because of their parents conflict not because their parents are no longer living together. Marital conflict has both direct and indirect effects on children.
Kids imitate their parents.
Psychologist, Rex Forehand, studies the social and academic adjustment of adolescents with divorced parents and with those whose parents remained together. Children exposed to parents quarrels and aggression learn those methods of dealing with conflict and frustration. Instead of learning from their parents mistakes, they will imitate them. I have witnessed this with a very close friend of mine. The father cheats on the mother, and the mother takes it.