I don't see what's wrong with calling asexuality a mental illness. If you take away the stigma of the term "mental illness," that's exactly what it is, same with homosexuality. It's a lack of a normal human trait beyond that persons control. Otherwise, it's just celibacy.
Except no one calls homosexuality a mental illness. In fact, homosexuality was explicitly removed from the list of mental illnesses and is no longer considered one. If an uncommon trait beyond a person's control were all that were required, then Einstein would also be mentally ill, because he was a genius. But even if asexuality were voluntary, it wouldn't necessarily be the same as celibacy. Priests are not voluntarily asexuals; they still experience sexual desire, only they control it through their vows. It is not their desire that matters, but the will of God. Also, not all asexuals are celibate. Some do engage in sex for social reasons.
I understand. I don't want to sound like a bigot on this issue because I'm not. I'm simply stating by saying asexuality or homosexuality, or better yet transsexuality, isn't a neurological condition (and likely caused by a genetic mutation of some sort), you're basically saying it's a choice. It's not. It's a biological error beyond a person's control.