It's immoral to cause and perpetuate unconsented suffering. Procreation causes and perpetuates suffering. Nobody had a choice on whether they were born in a world of suffering. Procreation is a violation of consent, and therefore it's immoral.
Defining this world as a "world of suffering" adds a subjective weight on that argument. It's a world of suffering and also a world of joy. Both of those things exist. In many countries, the average self-reported happiness and satisfaction with life out of a scale of 0-10, is in the 8-10 range. Very happy, in other words. So in those countries, it would be much more probable to have a child that is rather happy throughout life, than a child that is rather sad throughout life. In many troubled areas, it's a different matter, about 50/50 or somewhat less.
One could also argue that depriving other people of choice, is immoral. Taken to absurdity, for each potential child that we
don't create, we take away their choice to live in this world that has a greater than average chance of being happy. Therefore it would be immoral not to have as many children as possible. And that's obviously a terrible argument.
One could also argue, along with that idea of choice, that someone can commit suicide if they truly hate their life in this world. For the vast majority of people I certainly wouldn't recommend it. But technically, that's an option available to them. So by having kids (particularly in developed countries where the people having this discussion are), one brings a person into this world that offers a rather high probability of being fairly happy. And in the low chance that the child ends up being terribly sad throughout life for one reason or another, they have a choice whether to stay in this world or not. The vast majority of people choose to stay here, about 99% or so. The other 1% or so choose not to.
I note that people who argue against increasing the number of humans very rarely begin with themselves.
I don't plan to have any kids.
A more relaxed version is to just follow basic Kantian ethics of not doing something unless you think it's a good idea if everyone acted that way, within reason.
So one could say, for example, having 3+ kids, if everyone did that, leads to exponential growth on a finite planet, which is probably a bad idea.
But, having 2 kids or less, if everyone did that, leads to a roughly static population or a mild ongoing reduction in population.