I guess you can look at it both ways--not all two parent homes are stable, nor are either ready to be parents.
I've seen some single, stable happy families, and some not so happy. And, I've seen some families with two parent families that are stable and happy, and then, some not so much.
But I don't think anyone should be denied the chance to have a child in their life based on their marital status.
However, in an ideal world only the people who had the best intentions would get approved and provide a wonderful life for their child--single or not.
I've enjoyed reading everyone's responses--what a good topic to discuss!