I think I've worked out why I can't believe the character of Saga.
I think it's because she's obviously really high functioning, very intelligent, and even if she were autistic, I'd imagine (and I could be wrong) that she would have figured out a heck of a lot more about social interaction than she actually appears to have.
Women in particular with autism are often incredibly well adjusted socially. It might just be because they have fantastic coping mechanisms but for her to walk up to a bloke in the bar and say 'do you want to have sex?' wuld suggest she's not observed any other human beings at all, ever, in a similar situation - I mean it's possible, or maybe she just doesn't care about looking normal to people, but to me that speaks more of a huge amount of emotional damage, and a 'defensive' way of interacting, than autism.
I don't know if I'm right. But it pisses me off a little bit as being on the spectrum, I tend to try too hard if anything, not to say things that others wouldn't, and to mimic what I regard as 'normal' social interaction.
Also she never smiles and never appears to try to integrate, to look 'normal'. I think this is more like someone whose parents damaged them so very much (and I can see how, in her mother) that they have effectively cut themselves off from emotional interaction entirely as they can't trust it.
That's just my take on it. 
My mother isn't like hers but there have been elements of it over the years and when she is behaving weirdly, I go very monotone on her, very direct, no kisses on emails, facts and nothing more. It's a way of keeping up a wall against someone you cannot trust.