Disabled + Disabled =/= Disabled
I know someone whose parent's were both mentally disabled. She turned out as a normal human being.
If you argue the "They can't take care of a child!" point, no, they didn't. Her aunt did for awhile, but her parents kept very close to her and she knew who were father and mother were. There isn't anything wrong with her and she looks like an average, 5 or 6/10 human being. She's a potential one night stand that you'd consider to call again for a second hook-up.
ITT: Bigotry and eugenics at it's finest.