For example, homosexuality is rare and decreases the chance of having direct descendants. But would that qualify as an illness or a sickness? In some parts of the world being homosexual can result in a significantly reduced qualify of life, but is that a problem with the person or with society?
Now switch homosexuality for some far less socially acceptable (at least in Western cultures) attractions. How does things change?
Now, say we count out humans for a second (so messy legal/moral concepts like consent are less involved), and apply the thinking to different species of animals. When does abnormal behavior become illness?
Look at honey bees... most of the colony doesn't reproduce and takes roles that limit any ability to directly pass on their genes, but that is how the species works.
The difference between normal and abnormal, well and sick, mentally healthy and mentally ill are problems with not just autism, but with abnormal psychology in general.