Why would anyone give birth to a baby with Tay-Sachs?

I am against abortion in the vast majority of cases, including most cases with disability, but I think this would be an obvious exception. This isn't like Down's Syndrome. Down's kids can have a happy life. It probably won't be a productive life but their life can be worth living. A baby with Tay-Sachs has no chance at any happy life. They basically spend their whole life in extreme agony and then die some time before age four. I just think it would be really selfish to let someone go through that. They are much better off being dead. I think it's wrong to even let yourself get pregnant if both you and your partner have the gene. I mean, really, is adoption such a horrible proposition?