I would like your definition of a single parent. Do you mean single as in never married to the child's parent or do you mean divorced? If divorced, are you actively involved in the child's life or are you just a cash cow? I speak from being divorced from my son's mother. She divorced me because she wanted to be addicted to the internet and became a sex addict. I am still involved in my son's life as much as possible. However, his mother is telling everyone that she is a single mother.
But to answer your question, being not married and living in the same household as your child is very damaging to the child. A child needs both parents to learn from, not just masculine or just feminine traits. A child needs to see love from his parents and how to treat others.
I really believe parents should thing long and hard before they become parents to make sure they are going to be together for the long haul. You are supposed to get married for life and, even though my wife divorced me, I would still be married to her because I believe in it.