I don't think that's true, but perhaps it depends on what you mean by "rarely".
Any children coming into the world who are significantly less shitty than the average person will inspire social change that is larger than them. I think we could go so far as to say that's the better half of the population.
Kind of how a vegetarian (despite contributing to animal cruelty through milk) can do net good, even for animals, by inspiring others to eat less meat. Maybe they cause X units of harm, but their influence on others around them prevents 10X units of harm. The same can apply to vocal reducetarians.
That is not to say that you can complacently do bad things as long as you convince other people to stop doing the same kind of bad things you're doing, and that makes you a good person (that would be a moral Ponzi scheme). We should stop doing bad things ourselves too. But the effect on the world can still be a net good regardless of the personal moral failings of those causing the effects. Bad people (in character) can still be good for the world by none of their own moral progress but simply by the accident of their starting points.
It's probably fair to say that the children of the worst half of people (think racist anti-healthcare global warming denialists etc.) will probably still have a pretty negative effect on the world and drag down progress (not all will, some will break free of that, but a lot of them still will)... but these are also the people who aren't going to consider not having kids because of the negative effects on the world because they think they're warriors for God or something.
So it doesn't really play well to say that abortion is immoral if and only if you're a good person since either people mistakenly think they're good or don't care that it's immoral if they're really bad.
@PsYcHo How's that?