The negative utilitarianism states that reduction of suffering is more important than increasing of the happiness. Namely:
"reducing negative well-being is the only thing that ultimately matters morally".
Maybe I am looking at the problem too globally and maybe I should look at it from the perspective of a given individual. However, I believe that in the long run the procreation can lead to both reduction of suffering and increase of happiness.
This a little exaggerated example but here it goes. With each birth there is a slight chance that a person who will invent cure for cancer has been born. And of course such event would reduce a lot of suffering globally. And this example can be extended to other inventions that would have similiar effect. Perhaps in the end due to technological development mankind could erase all its suffering and keep some happiness.
I belive the reasoning behind negative utilitarism is that if mankind stop procreation then it will die out and that in turn will eventually lead to a point where there is no suffering at all. ( And no happiness )