One of the main criteria of a "disorder", as per psychiatrists making a diagnosis via the DSM, is that it causes significant disruption in one's life, and distress or dysfunction. Asexual and demi people can be perfectly happy being the way they are and experience no distress about a lack of attraction beyond feeling confused or different. In contrast, I doubt many people with OCD are happy being stuck turning a light switch on and off for an hour because their brain tells them they aren't doing it the "right" way.

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.