Some writers say having children is simply a very intimate and personal affair, so not the sort of thing constrained by morality. But I don't see the logic there--is there really a DMZ (demoralized zone), untouched by ethics? On other accounts, to say someone ought to have a child, or ought not, improperly limits their freedom, even if these are moral and not legal "oughts". But if moral "oughts" limit freedom (do they?), they all do. What's so special about "oughts" pertaining to reproduction?
Perhaps we can shed some light here by noting how we think about the ethics of self-preservation, and then connecting the dots between reproduction and self-preservation. So--first self-preservation, then the dots.
Jane is loafing around in the plaza drinking coffee, as she does most days, but today she has the misfortune of sitting near Dr. Wonderful when he goes berserk. Dr. Wonderful has his gun aimed at Jane, but she's armed too, and a good shot, so could take him out. Also in her pocket is a time-suspender. After pressing the button, Jane begins to gather information and reflect. Dr. Wonderful has 10 wonderful children and is on the verge of discovering the cure for a terrible illness. He's just off his meds, so if he shoots Jane, he'll be acquitted on grounds of insanity, and he'll go back to his wonderful research and his wonderful kids. Jane, by contrast, will go on wasting time drinking coffee in plazas. The world needs Dr. Wonderful much more than it needs Jane. Jane pushes the button again, returning to the fast pace of real life, and lets herself be shot.
She may not be wrong to make the sacrifice, but it doesn't seem like she has to. In fact, she's failed to appreciate her own right of self-preservation. She thought about the situation in a neutral, third-person way, but could have thought about it in a biased, first-person way. When our existence is threatened, we're entitled to that ... aren't we?
Let's not say we're entitled in an absolute sense, though. Put the whole scene on an airplane. Dr. Wonderful and Jane have both made it past airport security with their weapons. I don't think Jane can fire back if it means shooting a hole through the plane and bringing down everyone on board. But she can kill Dr. Wonderful in the plaza, even if she realizes that letting herself be killed will be better for all, from a third person perspective.
Okay ... but now suppose you're the fat man. Do you really have to jump down onto the track and let yourself be hit, to save the five children? That's going too far! If you're the fat man, there's a right of self-preservation that allows you to think about this in first person terms, even if (perhaps, perhaps), a bystander ought to look at it third-personally.
Time to connect the dots. At a genetic level, and on a psychological level, having a child has much in common with self-preservation, even if it's not identical to self-preservation. Yesterday I heard a Libyan woman interviewed on the radio. Her husband was Libyan broadcaster Mohammed Nabous, who who was killed by a sniper while speaking to his wife on a cell phone last spring -- she was 7 months pregnant at the time. She had wanted to be out there reporting with him, she tells the interviewer, but knowing their child was on the way made him able to face death. He wanted her to stay safe. On some level, having a child is surviving--at least that's how many people feel, and on a genetic level, there's a least a kernel of truth to their feeling.
So? So when we think about our reproductive choices, we don't have to have strictly third-personal thoughts about over-population or the problems of the elderly, just like Jane doesn't have to focus on total good, and let herself be shot by Dr. Wonderful; and the fat man doesn't have to have strictly third-personal thoughts and jump. If reproduction is erstatz survival, the right to self-preservation applies (more or less) to reproductive decisions as well.
But like in Jane's case, not absolutely--she shouldn't fire back on the plane. I can imagine situations in which the prima facie right to reproduce must give way to something overwhelmingly important. It also seems far more coherent to defend having a first or second child, as self-preservation, than to defend having a 10th or a 20th child that way. After a few kids, you've done your ersatz surviving, and that rationale for having a child must surely lose its force.
Now you might say: ignotum per ignotius (the unknown through the more unknown). Why is there a right to self-defense? Who knows, but any approach to ethics that says otherwise won't have much connection to the way people really live and think about their lives.