Baruch Hashem, neither I nor anyone in my close family has been diagnosed with Crohn’s. I do have a friend who suffers from it terribly, and have a small recognition of how dramatically it can affect a person’s life.
It is exactly the seriousness of the disease which could prompt a person to reject a shidduch on the basis of this diagnosis.
I do have a child with a different genetic abnormality, and was recently told by the geneticist that, even in the general population, one in every six people has a genetic abnormality of some sort (of course, some are more damaging than others). Obviously, the incidence is much higher among Ashkenazic Jews, who are effectively all intermarried with each other many times over.
It is time we got past the stigma of the words “genetic” and “hereditary.”