People with certain mental illnesses are prohibited from getting married.

Only not for the “preservation of the genome”, but because you don’t use vegetables to make yourself a sex toy. Vegetables may not like, and can not object, only complaining washes. It, indeed, will be grieving, because the body has been formed as standard and is experiencing a constant painful need, but it is at least not as terrible as giving to someone's arbitrariness the hell won't tell you what fantasies.
And this is not due to specific diseases, but to deprivation of capacity for aggregate severity. Yes, acquired (meningitis the same) - too, although it does not affect genes. It is not necessary to attribute the level of thinking of the grandmother on the bench with fascist mockups such as "they are psychic, they were forbidden to do children!" to the current legislation, and much more it is not necessary to justify some other fascist mockups by allegedly existing analogues in the legislation.