"When is it permissible to inflict suffering on another individual without their consent?"
If the answer is "never" then antinatalism follows. Apologists of pro-natalism face an awkward balancing act of justifying the imposition of harm in some cases while denying it in others. Thus if the question is not answered with sufficient care then unintuitive consequences can arise. Perhaps for some people there may not be any way to answer this question without unknowingly violating some other principle they took for granted. Most justifications of antinatalism use this fact to their advantage.
When this question is presented in the context of antinatalism, answers are frequently given with the sole intent to avoid the conclusion of the wrongness of procreation; answers that wouldn't arise if the question was considered in isolation.
For example, one might take issue with what precisely is the cause of harm inflicted on someone in the morally relevant sense. Someone who has children might be absolved of any hypothetical wrong doing if they weren't directly responsible for the harms that befall their children. However, arguing along these ontological lines is risky because it doesn't attempt to reconcile the harms with any benefit so could just as well apply to a world that contains only suffering.
Outside of the antinatalism specific arguments, the primary responses to this question are the following:
1. In self defense.
2. For punishment.
3. When the individual's psychological history suggests that consent would probably be supplied if it could be obtained.
4. When the one suffering receives something of sufficient compensation as determined by the past beliefs of the individual.
5. For religious reasons.
6. When the infliction of harm serves some greater good.
7. When the individual will probably believe in hindsight that the suffering was beneficial or a necessary evil i.e. retroactive consent.
8. When the suffering is outweighed by some quantity of pleasure or another good of which that suffering is a condition.
The first four don't concern antinatalism. I have nothing to say on religion other than that it can be used to justify anything and two people with different religions aren't going to agree on everything. The case of greater goods is like that of religion.
For retroactive consent, the question of harms can be thought to reduce to one of technology or of cunning psychological manipulation. We might phrase this in the form "what can someone be made to believe?" for anyone with sufficient technology can make anyone believe anything. Retroactive consent is problematic for this reason.
For the last and most common response, the goodness of something is tied to personal beliefs and is like the case of retroactive consent.
The key to arguing antinatalism lies in weaknesses of these responses but the whole issue is still very murky. There are numerous subtle issues that I don't seem to be able to conquer all simultaneously. However, I do see merit in the way the question can be used to focus attention and to expose and clarify the ways in which assumptions must be introduced (often artificially) to avoid the conclusions of antinatalism.