To diagnose KTK's central mistake: it looks like he is confusing temporal with modal absences, or non-presence with non-actuality. He writes:
[Causing a birth defect] is different from the case of a person who is maimed or otherwise subject to limitations in life after they have entered the moral realm - that person has actual interests that are thereby frustrated.
Granted, we might say that an aborted fetus is a "potential person" who is non-actual. (We might think of him as existing in possible worlds other than our own.) The fetus could have grown into that person, if brought to term. But given that it actually won't, the interests of this merely "potential person" need not be given moral weight. The person does not actually exist, and so has no actual interests.
But not all fetuses are like that. Some will be brought to term, and develop into actual persons: persons that exist in the actual world. There's an important difference between actually existing in the future, and merely possibly doing so. Actual persons have actual interests, even if they exist in the future, and we need to take those interests into account. Although it's permissible to refrain from bringing a merely potential person into existence, there is something morally bad about damaging actual (albeit future) interests. It is bad to harm an actual future person, i.e. make their life to go worse than it otherwise would have. Handicapping them as a fetus is obviously one such way to make their life go worse. So handicapping those fetuses that will grow into actual people is morally bad. KTK fails to realize of the handicapped actual future person that she too "has actual interests that are thereby frustrated."
It is obviously possible to act in ways that will make a future person's life go worse, even if they don't exist yet. It should also be clear that we can have moral obligations to refrain from such actions. (Our obligation to future generations is precisely why environmentalism is so morally pressing!) For a crude example, it would plainly be immoral to magically attach a time-bomb to a fetus, wait for it to develop into a person and then *BOOM*. Less crudely, you might use a very slow-acting poison. Either way, it's no defence to say, as KTK does, that the fetus has no interests and therefore you can do whatever you like to it. You have to think beyond the fetus, to the actual future person who is or will be* made worse off by your actions.
* = It's a tricky question exactly where we should temporally locate the harm. Is the future person harmed now, or does your present act cause a later harm? This may be a curious philosophical puzzle, but whenever it should be located, there's no question that the harm in question does (at some point or other) occur. So the metaphysical puzzle has no real ethical import, at least on the issues we're discussing here.
(Here's a trickier case: what if you handicap a fetus, which the distraught mother for that reason decides to abort? Given my theoretical commitments outlined above, I must say that you didn't harm the potential person who would have been born were it not for your interference. For as it happens, they are not an actual person; they do not actually exist in the future, and so they have no actual interests. Instead, the harm you do is solely to the mother: you deprived her of the child she wanted to bear. Alternatively, if the mother doesn't care either, but the future is made a worse place than it otherwise would have been -- perhaps she has a "replacement" child whose life goes not so well as the aborted child's would have -- then this could be a case of badness without harm.)
KTK supports his misguided conclusion with two similarly misguided arguments. First, he writes:
A life with [disabilities] is still better than nothing at all... So such an infant has suffered no harm, even if it is disappointing or frustrating that it could have had a better life.
Sure, it's better to be born than not. But of course that merely shows that birth is no harm. It's worse for a person to be fetally handicapped (or poisoned, or wrapped with explosives) than not, and that shows that fetal handicapping (or poisoning, etc.) is a harm.
Second bad argument:
We do not blame parents of children with accidentally-acquired birth defects, even when those parents deliberately choose not to abort that fetus. The choice to create birth defects and then bring the fetus to term is essentially the same act.
Um, no. The parents in the first case have no option to make their child healthy. The choice is exclusively between a disabled life or no life at all. The relevant act here is simply childbirth, and we've already seen that that is no harm. In the second case, by contrast, the parents also have the choice to refrain from handicapping their child, and allowing her to instead enjoy a healthy life. There are two very different actions they could take to prevent this. One would be an abortion, so that she (the potential person) never actually exists at all. That's fine. The other would be fetally handicapping the child so that she lives a less-healthy life. That's not so fine. The relevant act here is imposing a disability on an otherwise healthy person. There's a certain lack of discernment involved in claiming that childbirth and gratuitous handicapping are "essentially the same act." I don't mean to belabour the obvious, but we really should take care to distinguish the two.