Regardless of population concerns, when you give birth you force (eventually) sentient beings into existence without giving them the chance to consent to existence. Even if they are born into a perfectly happy family with all the means to raise them, these kids will be forced to suffer the pain of being alive, either through depression or simply the pain and fear of death. The only way to opt-out of living after birth is through death, and even if death is through painless suicide or even unforeseen accident, there is always the fear of dying which is undeniably unpleasant.
Assuming that inflicting suffering on beings capable of feeling pain/fear/what-have-you is immoral, how can having children be moral, even if there is the possibility the net joy they experience is greater than the net pain? Isn't that risk something that should be consented to? Or does the preservation of humanity outweigh that facially immoral act?