That's a good question. I believe if a mental, physical, or psychological trait hinders your QOL or impedes your ability to perform basic tasks, it qualifies as a disorder or dysfunction.
I think everyone has traits that make them lean towards dysfunction, but they are manageable, tolerable, or not seen as abnormal.
Most Deaf people don't see their deafness as a handicap or dysfunction. While some hearing folks might think "they're missing out on so much" it can't be further from the truth.
This was the first time I saw Asexuality mentioned on a major community, so I was happy to see the spread of awareness, then the first comment was along the lines of "it's probably a mental disorder". To me, that sounds anti-LGBT.