Something similar has come up before here, and while I can understand the anger and disappointment in those who have experience of living with someone with Down's, it's still a huge responsibility on two people, to look after and basically devote the rest of their lives to caring for this child.
I personally have always had a phobia about the mentally disabled, and it needs to be understood, I've not chosen to dislike them, like racism, it's more an irrational fear, like a fear of bees or open spaces. However, a bee phobia gets you mild ridicule for flailing around at a tiny bee, whereas my avoidance of any situation where I'll be stuck having to interact with someone with Down's is treated with disgust as tho I'm just choosing to be crappy to someone less fortunate.
We've got to 4 pages already, how about a twist on the original question... Would it be wrong to, if it could be detected in the first few weeks of pregnancy, abort all babies with a serious mental disability, enough to mean that they'd never be able to function in the world alone without help?
As for the thing of the couple really wanting a kid, go adopt, there's thousands of young kids needing a loving home, I wish we could shake off this caveman attitude that only our own biological spawn 'counts' as 'having a baby'.
The way I see it, as loving and gentle as people with down's can be, surely no-one would wish it on a baby, therefore either trying again, or adopting would be a sensible option.
Please note I don't think anyone is asking that we kill anyone with Down's, just that we try to prevent it happening in the future.