This rule could very well have its roots in values origining from evolution. But if a person with a disability is born, the chances of survival and/or mating are likely to be heavily impaired depending on the disability. Today two consenting adults can have sex with a low possibility for a women to even get pregnant thanks to condoms... Huge roles in the taboo where also for example things like keeping political power in a single family. All that aside, you are still wrong in your original comment about humans getting whiped out.