In terms of its not being fair to decline to marry someone because they have a quality that they can’t help, I disagree with that premise.
Would you marry someone who has bad middos because he can’t help it? Usually when people have bad middos, it is not their fault – it is the result of nature and nurture. People don’t choose to have bad middos, and we are supposed to be “dan l’kaf zchus” that everyone is doing the best with what he is given.
Basically, you can say that about every trait – that it is not his fault so it is unfair to decline to marry him because of it.
We are not supposed to just marry anyone. We are supposed to choose whom to marry based on the traits that they have and determining if these traits would make them compatible for you.
We have no way of knowing who is beshert -that is exactly the way in which we try to figure it out – by looking at their traits and thinking about if they have the traits that would work for you.
And the difference between a physical ailment and an emotional one is that an emotional one does often affect the person’s middos/personality traits (perhaps a physical one can as well, and in that case, that may be a legitimate reason not to go out with the person). Just like you wouldn’t want to marry someone who has bad middos but doesn’t have a syndrome to blame it on, why should you want to marry someone who has bad middos just because he can give it a name.
According to your logic, there is no reason to ever say no to any shidduch, and shidduchim should just be made randomly. There is a story in the Gemara about someone who tried to do that, and it was a failure.