That's a not a totally fair comparison. If we barred deaf people from reproducing they would lose that right completely. When we outlawed incest that only eliminates maybe 10 or 20 people a given person can reproduce with.

Another way to look at is that deaf people have no choice in the matter, whereas someone contemplating incest does. Kind of like deaf people can drive, but that doesn't mean the rest of us can drive wearing headphones.

I'm not saying it's a great law, but it isn't without some rational basis.