A crutch is something you use to walk when you are unable to do so in the conventional, two-footed way.
Last week, my autism diagnosis was called a "crutch" by somebody very close to me.
It hurt.
A crutch is something to lean on when a part of your body is unable to work as society deems "normal." It is something you can use to explain why you take forever to get to class, something you can use to explain why you can't take a ton of stairs. "Sorry, I'm on crutches," you say, and you are taken to the nearest elevator.
If a crutch helps you get around, why was the word used as an insult?
I had abstained from talking to the offender for a day because I only saw them in times they typically liked silence. They were angry that I didn't recognize this, I explained I had autism, and they shot back that it was my "crutch."
The message being conveyed was, "you're using your diagnosis to excuse your failed interpretation of social norms that come off as rude." Yes, that's how autism works. It comes with social disability (note: the term Asperger's syndrome was redacted by the American Psychiatric Association in 2013 and is generally considered outdated). Explaining myself helps people understand why I can't do certain things like make eye contact or wait my turn in the conversation. It's not because I'm lazy or a jerk, it's because my neurotype doesn't find these things as easy as yours does.
Think about crutches in a literal sense: "You're on crutches to excuse the fact that you can't walk because of your broken ankle." Yes, that's how crutches work. You use a crutch because your ankle can't work the way most ankles do. Without the signs that your ankle doesn't function normally, people would really wonder why you were trapped on the first floor of the elevator-less student union for hours.
Maybe my label is a crutch, but not in the insulting way my family member meant it. I use my diagnosis to explain why I act the way I do. When you see somebody with crutches, there is a visible cue that the person will struggle with stairs, and you don't judge them for it. My saying I have autism is an explanation of why, through no weakness of my own, I cannot act in more convenient, socially acceptable ways.
By no means am I likening autism to something as inconvenient as breaking a bone. Believe me, it has so many benefits, too -- hyper-focus, synesthesia in some cases, intense joy that comes with special interests. It also should not be "cured" with time or medicine like a broken bone can. But the connection here is that labels are typically used as explanations, not excuses.
After all, when you use real crutches to help you get around, people assume that you need them. They don't think that you're weak for taking care of yourself, right?