It's saying that if we terminate base on a disability we are preventing that person from a chance a life based on the assumption that it would be better not to exist rather than have the disability in question. Those born with these disabilities might disagree and they might see such an action as a judgement against their own existence. This further adds to their suffering.
Once we are talking about counterfactuals, that is things like "would haves" about people who don't exist but could have, it opens the door to considering other people who don't exist but could have.
Under the assumption that a family who proceeds with the Downs foetus to birth has fewer children afterwards, ought we not to consider the later children who might not exist as part of this moral equation too?
What if a family who proceeds with the Downs foetus to birth is likely to have multiple fewer children after than they would have had?
Some people born with disabilities might disagree - and have every right to disagree - and see the action of abortion as a judgement against their own existence. But in doing so, are they not identifying themselves more with the counterfactual Downs foetus than with the counterfactual non-Downs foetus?
If so, does that not suggest people who think that way identify themselves with the disability to some degree ("In that world I would have been aborted"), rather than identifying themselves as people independent of a physical characteristic ("In that world I would have been guaranteed my body would not have this disability")?
