Quite simply this is none other than a most heinous form of narcissism. The issue is not whether deaf people can have an enormously rich and meaningful life. Obviously they can. They can live a life so rich and meaningful that they are not mindful of their deafness. Indeed, it is impossible that a deaf person may succeed in ways that he would not have succeeded has he not been deaf... [But none of this changes] the fact that by and large hearing is an extraordinary asset. It is precisely because it is such an asset that we marvel at people like Geoff Herbert; for he flourished mightily without it. More accurately, he flourished mightily in spite of a considerable biological disadvantage. He has not shown that there is no difference between being deaf and having hearing. Not at all. Rather, what he has shown is that it is possible for a person to surmount that biological disadvantage with considerable majesty...
Just as there can be no excuse for treating the blind or the deaf as lesser human beings—as surely they are not, there can also be no excuse for turning this truth into what it is not, namely a license to ignore the reality of the difference between a body all of whose parts are functioning properly and one where this is not the case. To render a child deaf or blind at birth is to make it the case that a child is born with body parts that do not function properly. There is no amount of success on the part of any deaf or blind person that defeats this truth.
Does 'proper function' have any intrinsic normative significance, though? Suppose that, for whatever reason, deaf children could be expected to live better and more successful lives than children whose ears worked 'properly'. (Imagine some fantastical affliction that spreads via sound waves, and so affects only those who can hear. Or a world filled, inescapably, with the sound of fingernails on a chalkboard!) Deafness would then be an advantage, and presumably perfectly permissible to gift to one's children, no matter the subversion of biological proper functioning. Indeed, I'm tempted to say, of such a world, that it would be impermissible to intentionally bring one's children to be of able hearing!
It's tempting, then, to think that the morally relevant factor is simply the expected impact on the quality of the child's life (and the impact on others, if there are significant externalities). But here's a trouble case: what if the "disadvantage" is socially constructed, and only arises because others act unjustly? Imagine, for example, an interracial couple in a racist society. Because others in society are racist, dark-skinned children are at a considerable disadvantage. Does that mean it is impermissible for the couple to intentionally bring it about (through genetic screening, etc.) that their child be dark-skinned? (Maybe they think this will help their child 'belong' most fully to the black community, which the parents value so.)
Some deaf people want to claim that this is precisely their situation. Is it? If so, does it follow that it's permissible for them to have intentionally deaf children after all?