The world is full of suffering. It is our duty to reduce suffering. The simplest most effective way not to increase the total world suffering is not to procreate. It is better to stop the problem before it even starts.
This is the fundamental tenet of antinatalism. It is refined further by considering it at the level of the individual where particular circumstances might be cited to prove that suffering will be the lot of any child that is born. The parents may be living in poverty. They may know that they have particular inherited disease. It is unethical for them to have a child which they cannot care for and which may be afflicted with illness, and which ultimately faces death. This is local antinatalism.
At the global level the sources of pain and suffering are more to do with overpopulation and the changing environment. It is unethical to bring a child into a world ravaged by climate change. There is the possibility that the human race will suffer extinction as so many other species have. Again antinatalism offers the possibility of a dignified exit from a world of pain.
The argument against the antinatalist view is essentially that we cannot know the future, either at the individual or at the global level, so we cannot assume that it will be bad. Well, not all bad.