Sounds as if she stumbled onto the AsPartners website. I agree it's pretty hurtful to be seen as a potential monster. My ex-wife did something similar, I can't remember the details now, I think she figured I'd given her Cassandra's syndrome because she'd read a bit of intense venting about an "ASH" by a neurotypical lady whose relationship wasn't doing very well. I was stuck in a double-bind, because every effort I made to argue my case was seen as invalidating her feelings. As far as I can tell, the truth of the matter was simply that our values turned out to be way too different for the pain of the relationship to be worth the pleasure.
In your case I think I'd try asking her what exactly she thinks you're likely to do that she's so scared of. Read the details in the horror stories that have scared her and see what you make of them. I'd also ask why, if your behaviour has been OK until now, she expects it to go that way. It's not widely thought that ASD gets worse over time, but maybe she's expecting the relationship to demand more social skills in the future.....I think that can happen, relationships are often very easy for a while, and get more difficult later. But that danger isn't limited to AS-NT relationships, it's pretty universal. If two people are genuinely on each other's side and are willing to work through problems, it's possible to keep things good.
Most of what I've seen on those websites has been pretty irrational, and (their) anecdotal evidence isn't hard evidence, though lay people often find it surprisingly convincing. Sometimes it's important to let people vent, but when I see a website that does nothing else, in public too, I smell a rat. The idea of healthy venting is, I think, to relieve pressure, not to reinforce and perpetuate bigotry. There was no mention of the NT partners' own shortcomings. It's easy for some people to prop up their own self-esteem by criticising others who don't get the right of reply. It's hard to see what else they might be trying to achieve there. I don't recall seeing any NT men there. I've no idea why that is.