In fact, you may agree with some of their points. For instance, if a fetus is destined to live a short and painful life, suffocating in it's own bodily waste products upon birth due to a congenital abnormality, you may agree that it should be aborted.
A lot of these philosophical positions are just an excuse to question some of our deepest held beliefs and reveal our own prejudices and misconceptions. Some people think that giving birth to a child is a gift and the child should be indebted to the parents for the rest of their lives for that gift. Anti-natalism shines a cold light of truth on this misconception.