Of the many arguments that bolster antinatalism, I'm contemplating only consent here:
Consent: The fact that life contains suffering might be tolerable under certain circumstances, i.e. if one could choose they want to live such a life. Actually, we are all here without our consent. Our parents condemned us to suffering and death because of their own, personal wishes; no one can deny that. (Forced marriage and pregnancy in some regions of the world is the obvious exception. Still, it is people other than the parents who are responsible in that case.) Consent is an important moral principle, though. It is the reason why it is illegal to sexually abuse a drunk person, or to produce child and animal pornography.
One might now argue that most people do not complain about their birth in hindsight. Apart from the fact that they might do complain if procreation were not taken for granted by society, the argument does not hold moral value. When you approach a stranger in the streets, break their arm and then gift them a suitcase full of cash, they can justly sue you on grounds of battery. You can not argue that you wanted to benefit them on the whole. You have simply inflicted suffering on somebody without their agreement.
A lot of people counter the consent argument by saying that the unhappy could simply kill themselves. While that is true, it is problematic for several reasons. In order to entertain the idea of committing suicide, most people have to have experienced a substantial amount of suffering. On top of that, suicide is difficult to realize as it requires you to overcome your survival instinct, which takes much strength. Even if you achieve this, it is not easy to overcome your body. Jumping off a tall building, for example, requires additional courage, moreover such a method can end up traumatizing or injuring other people. Further, no method is really safe: Jumping off high buildings or bridges, shooting, poisoning, hanging, self-immolation, electrocution etc. are methods that can be survived. They all include the risk of ending up severely disabled in the worst case, resulting not in salvation but in a harder life.
Suicide is a social taboo – which also makes it more difficult. Were it seen purely as an alternative to living, and physicians performed assisted suicide, it would already help a lot. Instead, you have to keep your suicide plans a secret and rely on delicate methods. You are not granted the option to say farewell to friends and the like and usually die alone. A lot of people do not commit suicide because they do not want to force anybody to put with disposing of their remains once they are found. If suicide were accepted in society, you could die with medical assistance, pain-free and among familiar faces. You could easily donate your organs and therefore even help others.
Yet this summary doesn't counter this Reddit natalist's argument that "[i]f, at the time of the action, there is no someone then consent is not breached":
The point is that the concept of consent doesn't make sense when manipulating non-sentient elements. The physics of life creates sentience. The proposal of anti-natalists here is that making people occurs without consent of the person made, but it misses the point that people don't make people, people make sexy times.
The antinatalist then purportedly tries to transfer the consent to the newborn who obviously can't, didn't, and never can, consent to childbirth.
Many people make new people on purpose (some even get medical assistence [sic] to accomplish it). I grant you that many also are made unintended.
The main problem occurs when the offspring realise they are trapped in a system they have little or no control over, which is very hard to get out of. Some don't agree with being subjected to the unneccesary, sometimes constant suffering lives involve.