It's something you really couldn't understand unless you were deaf yourself. Many straight people do not understand why so many gay people would not be straight if they had the chance, so this is really no different. Being deaf in a hearing world is just as unique as being gay in a straight world. It sets them apart from the crowd. Plus, the original topic of discussion here means that they have their own language that realistically is useful to everyone. Think about it, how many times have you been out with a friend (who is gay or knows you are gay) and you spotted a hot guy whom you wanted to comment on? If the surrounding area isn't very gay-friendly then you can't. but if you guys knew sign language then you could and the other people around you would be none the wiser. Foreigners do it all the time since it is very unlikely for someone in the US to understand Mandarin, or for someone in Japan to understand Portuguese.
Also, deafness is no more a "disability" than gayness. Gay people are born gay. Deaf people either are born deaf, gradually lose their hearing for some unknown reason, or something happens in their life that kills their hearing. It can't be controlled (unless you are a musician). The only adjustments that deaf people must make is be bi-lingual and have a using alert system (for doorbells and telephones) and other minor odds and ends to be comfortable. Gay people need to be cautious with sexual partners, public affection, and much of society.
Building on that, deaf people are not completely deaf. The part of the brain that processes sound for a hearing person processes vibrations for a deaf person. This way, deaf people are more sensitive to vibrations than hearing people are. Guess what sound is? That's right, vibrations moving through the air as waves. When you play music on your computer or in the car, so the speaker not vibrate? Try this -- turn up the volume several notches or so. Does the table on which the computer speaker sits or the interior of your car not vibrate?
As for condemning the deaf couple who wants a deaf child -- Do gay couples not want gay children? Do French couples not want French children? Do African couples not want African children?
I'm not saying that your thought process is wrong, just that I completely disagree with it and consider it to be very ignorant.