I only know one couple who have not done so, and they decided not to for reasons of faith. They are Jehovah's Witnesses and they feel that bringing a life into this world is a little redundant when they can bring on into paradise after armageddon. Sounds like a well-thought out reason to me. I on the other hand, have full intention of becoming a parent. The ratio then, among those I am aware of, is pretty low.
My partner and I, for one. We both seem to be lacking in that parenting gene. In addition, my partner's nephew is a complete Devil Child, in both our eyes, and that seemed to confirm our belief that we'd rather have cats and dogs running around the house.