As I read it, anti-natalism is less about birth itself being immoral than it is about life being undesirable. The core idea is that it is better to have never been born - something many people must have felt at some point in their lives!
Certainly if one was never born one would not have to endure the many unhappy and painful things that life brings on. It is a good question whether - on balance - one is happy more than one is sad during one's life, if there are more bad things in life than good ones.
As each life contains more sadness than happiness it is immoral to create another life, condemed as it is to more pain than pleasure, because the net effect is an increase in sadness and pain in the world.