Flinching at another's touch is often regarded as discourteous or unkind, even if aforementioned flinching is involuntary.
This is something that I learned fairly early on. Neurotypical people just don't seem to understand that I'm not being rude, I don't think you're disgusting and I certainly am not doing it just to get at you. It makes me feel uncomfortable when you touch me, especially without permission, and I am under the impression that it is only human to not want to feel uncomfortable. All the same, I do try to explain to people who seem hurt that it isn't anything personal.