It’s impossible and that’s exactly the point. The fact that it is technically not possible to get consent from someone is certainly not a justification to act without consent.
Would we accept putting someone at risk of significant harm without consent in any other situation?
If it is not possible to get consent from someone regarding some action that is so significant and relevant to that someone, the right and logical thing is not to do that action, not to ignore the fact that consent wasn’t given and do it anyway.
The fact that it is not possible to get people’s consent for their creation is certainly not a reason to do what we want even though it will greatly affect them. On the contrary, this is precisely the situation where we should not take an action that will affect others so dramatically, precisely because their consent to it cannot be obtained.
There are exceptions to this, such as acting in a way that affects someone, without asking permission beforehand, in order to prevent major harm, for example if we noticed that someone is about to get run over. But this is morally justified because we act to prevent greater harm to that person, and reproduction is not an action that prevents greater harm, or harm at all, since someone who was not created is not harmed in any way by not being created, as there is no one at all who can be harmed by never being created.
Creating a person is a decision that someone will feel pain, illness, despair, frustration, anxiety, loss, fear, heartbreak, death, etc. The fact that the person who was created cannot give consent to all this, and to many other things, is not a justification to impose them anyway, but merely an evasion. It is clear to us that these things will be experienced by whoever will exist as a result of our decisions, and it is clear to us that causing someone such things requires their consent. We find the idea of causing someone even a fraction of this without consent when it can be given, clearly immoral, so causing someone all this without consent just because there is no technical possibility to provide it is an exploitation of a metaphysically ambiguous situation. Before our action there is no one from whom consent can be obtained, but precisely as a result of our action someone who must give consent to all the harms and vulnerabilities they will experience during existence, will be created. To justify not obtaining someone’s consent to all the expected harms, using the fact that before we created that person they could not give consent, is morally wrong. Especially since it is us who created this situation.
It is true that there is no one to ask consent from at the time the action was carried out, but since this action will certainly affect the created person in the future, this claim is nothing more than sophistry and evasion. It is clear that if the action is carried out there will be someone who will be affected by it and there will be someone from whom, under any other conditions, we would require ourselves to ask for consent, therefore there is no reason to ignore the problem of not obtaining consent in the case of creating a person since at the time of the action there was no person who could be harmed. A famous example given in these contexts is planting a time bomb in a kindergarten with a timer set for another 7 years. Although none of the children who will be harmed by the bomb were present at the time of its planting, we will definitely claim that whoever planted the bomb is guilty of causing suffering to the children who will explode in seven years.
The length of time between the moment when a harmful action was taken and the time of the harm itself is morally irrelevant, as long as the relationship between cause and effect is preserved. The decision to create a person exposes that someone to a multitude of vulnerabilities, a large part of which will occur for sure and some of which may occur at any given moment. The time the harm occurs and its distance from the decision is morally irrelevant, as is the question of whether there was already someone who would be harmed at the time the harmful actions were taken. What is relevant is the decision and its effects. And these don’t simply evaporate just because at the time of the decision there was still no victim but as a result of it, soon there will definitely be one.