I see antinatalists constantly struggling to crush an argument that's best summed up thusly:
Nonexistent people never get a chance to choose whether their individual lives are worth living or not. Because a nonexistent person cannot desire things, we cannot make statements regarding whether a nonexistent person will desire life if presented the choice.
Another variant might look something like this:
We don't know whether a particular person's coming into existence will be for the greater good or not, so we have no right to prevent it from happening. For all we know, a person's birth will be, at the minimum, good for the person himself.
An easy way to trump this is to alert the person making the argument to the fact that he or she is actually on step two in the line of questioning. The first step is:
Is life necessary?
If there is nothing necessary about life, then we cannot possibly justify it, given that stakes are present. We can only justify taking risks with stakes involved where it's necessary, or where the stakes are the lowest possible out of all the options. If the lowest possible number of stakes within a given scenario is zero, and the other options are not necessary, then we should choose the option with zero stakes.
Again, if you're not willing to roll a six-sided die with five amazingly pleasurable sides so long as AIDS or stomach cancer or the bubonic plague is on the sixth, please remember that every day, someone gets the "I just fell in love" side, someone gets the "I just won the lottery" side, and someone, somewhere gets the "Wow, I'm HIV positive" side. If you're okay with this but not okay with rolling the die yourself, then you are a hypocrite.
The issue at hand is NOT whether potential persons should be allowed to decide for "themselves" that their lives are good; it's whether there is a real, hard reason to fabricate the dilemma in the first place. I'm sure that ninety percent of the human population enjoys ice cream, but that doesn't give you the "right" to order a friend ice cream for dessert without first asking him if he wants ice cream. What if he's in the ten percent that abhors ice cream?
Now imagine that, not only does he dislike ice cream, but he's lactose intolerant to the point where eating even a single spoonful will cause him to vomit uncontrollably and become hospitalized.
Now imagine that eating ice cream is not of such dire importance that we can ever deem it necessary for anyone.
Hey, you haven't forced the dessert on him yet, so we can't say anything about whether he likes ice cream, right?