The central claim of antinatalists like me is that being born is not a good thing - specifically, that it's better not to be born than to be born.
Lots of folks have the same immediate reaction to first encountering this: "so why dontcha just kill yourself?"
I wish to illustrate here how this response is a non sequitur - specifically, almost identical to a response of "so why don't you marry it?" in response to me telling my friend how much I love Christopher Brosius perfume.
In the "Why don't you marry it?" case, the respondent is implying that the assertion of the speaker is not genuine unless the speaker is willing to give the most extreme possible evidence of his conviction.
Note first that this is a personal attack (ad hominem, as the kids call it these days) rather than a response to the argument.The respondent is not analyzing anything to do with the argument, but rather is questioning the sincerity of this speaker's protestation.
Second, he demands the most extreme evidence imaginable - marriage as evidence of love.
Similarly, when someone hears I think it was a bad thing for me to have been born, and then asks me why I don't kill myself, he's (a) failing to respond to any argument I have made, instead choosing to challenge my sincerity, and (b) demanding the most extreme imaginable sort of evidence for the claim.
This is a non sequitur because it is possible that it is a bad thing to be born, but that once born, it is worse to commit suicide than to remain alive for one's lifespan (worse for others one cares about, or even for oneself). I have heard at least one antinatalist assert that it's wrong to create new people precisely because death is so awful and scary to think about; we would not have this fear, would not experience this irrational negative affect, if we had not been born.
Also, similarly to the "Why don't you marry it?" example, it is not legally possible (to my consternation) for me to marry a bottle of perfume. Indeed, there are many barriers, legal, practical, and even moral, to committing suicide. It's like asking a starving person in Somalia, "if you're so hungry, why don't you drive to Wendy's and get a cheeseburger?"
One last note: lots of people get married for immigration purposes. Lots of people kill themselves who nonetheless think it was a good thing that they were born and got to see all those puppies and sunsets. The evidence demanded is not even necessarily evidence of the proposition asserted!