I'm an Aspie and don't consider my social deficiencies to be a form of being mentally challenged. I just see my brain as being wired differently than others. I excel at some things my husband is terrible at. He excels at other things I'm terrible at. It doesn't make sense to arbitrarily assign me with the mentally challenged label in that context. In protest, I reject the label :) If we met, you probably wouldn't even know I was an Aspie. I've been told I don't "look like" someone with autism. I don't know what that means, but I'm pretty sure it has to do with stereotyping and labels.
I also say "autistic". I don't care if Jenny McCarthy thinks it sounds like I'm calling myself cancer. Even if she is hot ;)
I'm not saying you're guilty of this, but for many of us (Aspies, Auties) it feels like other people label us and we're not allowed to even engage in a discussion about our own label.