While I agree generally, there are some high-functioning autists that are quite attractive.
Usually though, they aren't.
I'm guessing this could be for a variety of reasons. Including, but definitely not limited to, the inability to read social cues so they're often awkward IRL, and that may affect how you perceive them physically; hygiene habits - I've noticed a lot of autistic people aren't great with keeping up with hygiene habits, I'm not autistic so I don't know why; and maybe just lack of care about their physical appearance as they're not really interested in romantic or even social - to some extent - relationships.
That's just my take on it, though.