Just because the parents aren't together doesn't mean the father can't be in their life. I haven't been with the mother of my two daughters for like 8 years but stay an active participant in my daughter's lives. Just as damaging to children are parents staying together who are absolutely miserable with each other. They turn that sadness and anger at the situation towards their children and the children see the parent's fukked up relationship as the norm.
So it's wrong to say that if two people have a child together they need to stay together. The real reality is that people need to take responsibility and make their children a priority regardless of the relationship with the mother.