I made a post on this topic before, but it utilized Benatar's asymmetry which I have come to reject (see previous posts).
Nevertheless, I still consider myself an antinatalist of some sort, and I still maintain that antinatalism does not lead to pro-mortalism.
Setting aside other arguments for antinatalism, such as the awesome risk associated with birth or the utilitarian argument, I will focus purely on a pessimistic argument that would have been utilized by Schopenhauer and co. Basically, the sum the view up, life sucks.
Because life sucks, we shouldn't have children. Plain and simple.
However, a common response might be that if life sucks so much, why shouldn't we kill ourselves, or at least be suicidal?
There are a few ways the antinatalist can respond.
They can say that they really are just clinging to life. Barely making it through the day. They affirm that they really would want to die, if they could get around the inherent fear of death and the irrational impulse to continue to live. Many of the YouTube antinatalists come across this way.
However, I doubt that most people are suicidal. I doubt that most people actually wish to die (including these YouTube antinatalists). But how can a life worth living not be worth starting?
The trick here is that our lives possibly aren't worth continuing. But they certainly aren't worth ending either. There is no dichotomy here. We can all have mostly mediocre lives that don't warrant a creation of another one but nevertheless don't warrant the end of the ones that already exist. There's not enough pain to warrant a suicide. But life generally isn't that fun either. meh
Of course, this by itself "tones down" the fierceness of many antinatalists and their views. If life is merely mediocre, they it's not worth being born into but it's not a horrible, terrible tragedy either. The tragedy only comes along when life is romanticized.
It's only tragic when life really does suck in the horrible, terrible way. But since most of our lives are merely mediocre and not horrible and terrible, a better argument for antinatalism that maintains our general wish to continue to live (not being suicidal) is that there is an inherent risk associated with birth, and that the worst-case scenario is always worse than the best-case scenario, even if the best-case scenario is itself a good experience.