With childbirth, you know it's coming. With nut shots, it's 0 to #@$%!!!!! Also, women have anesthesia for childbirth so it doesn't even hurt in most modern countries. Nut shots hurt way worse if you ask me.
As I am not a female, I cannot give an equated answer to your question nor would I ever want to know the pleasure or displeasure of child birth. But, seeing as I'm a male and have, at least one time in my life, been kicked in the groin, I can say with certainty being kicked in the family jewels hurts like hell.
When women are giving birth doesn't their body release some oxytocin as well as some other chemicals to stop the pain?
It's actually funny you say that, considering the rewards of childbirth are, well, a child. While the rewards of getting kicked in the nuts? Exactly. Your comparison is flawed. Knowing what a joy and all out inspiration my child has been on my life, if I had to get kicked in the nuts to have another one, then it'll be a pleasure.