Philosophical Syncretism
Merging antinatalism with other philosophical frameworks
Moral anti-realism: the belief that there are no objective moral values
Even if this is true, there are still some fundamental principles that people abide by to survive and for the welfare of society. Therefore, there are some axioms that are presumed to be true when applied to behavior in the real world (see quasi-realism). Otherwise, causing significant harm to others is ethical, all atrocities are justified, and all laws should be abolished since people should not be punished if nothing is inherently bad. Antinatalism can be one of your axioms if it is agreed that reproduction is harmful.
“Since nothing is inherently unethical, why should I care?”
We have to create our own principles and act in accordance with what would be best for society and ourselves. Just because it doesn't exist naturally doesn't mean it isn't real. Nothing exists until we make it exist. For example, forests don't exist. It's a concept we made up to categorize something, but it doesn't technically exist in nature. There is no natural sign that says "This is a forest" because it's a concept we created ourselves. Same goes for every country, money, language, idea, or anything else that's ever been created. They have no inherent meaning or value except the ones we assign to it based on what we need or desire for it to accomplish. Just because they are abstract does not mean they aren’t real.
“But how will we know which philosophy is the ‘correct one’?”
You have to find one that agrees with your worldview and does the most to benefit society. Just because there is no one single moral philosophy does not mean that all of them must be false. That's why understanding grows according to what you learn rather than rejecting all of it.
Contractualism: the belief that moral principles are based on a social contract between oneself and society
Antinatalism can be part of this contract since reproduction inflicts harm on others and violates the very reason why there is a contract in the first place (to prevent harm to others in a society). If you only acted "morally" to prevent punishment, which is fundamentally selfish, would you like to be legally scammed or tortured by someone else like a government entity? If not, why would you find it ethical to do the same to others?
Virtue ethics: the belief that ethics are based on the virtue of an action
If you agree that suffering and inflicting suffering on others is not virtuous, then reproduction is not virtuous. Life cannot be inherently virtuous if it is nonconsensual and will lead to harm in the same way that rape is not virtouous.
Utilitarianism: the doctrine that an action is right insofar as it promotes happiness, and that the greatest happiness of the greatest number should be the guiding principle of conduct.
Utilitarianism means minimizing suffering while maximizing happiness. The best way to do that is to cease reproduction to prevent suffering and raising the happiness of those who are already alive. While that would also mean the unborn would not experience happiness, that is neutral as they never desired it in the first place as they were never born. It would be unethical to force one’s own desires onto others under the assumption they would enjoy it but would have no easy escape if they did not (see “Forced Hobby Comparison” thought experiment).
Positive Utilitarianism: the promotion or maximizing of intrinsic value
There is no point in maximizing pleasure in a being before it exists as they have no desire for it. Being born creates that desire. And since we do not know if the person who is born will have more positive experiences than negative experiences, we should not decide that they should be born without their consent as it could be a net negative on the world.