Is asexuality a mental illness?
Asexuality is not a mental illness and there are three primary reasons for this – no medical professions consider asexuality a mental illness, there isn't evidence of a link between asexuality and psychopathology, and on a conceptual level it doesn't make sense to label harmless self-identification as an illness.
There is no medical profession in the world that defines mental illness for asexuality. For example, the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) – which is used in the US for diagnosing mental illnesses – does not have an entry to define what counts for a 'diagnosis' or asexuality. The closest that the DSM-5 comes to diagnosing something similar to asexuality is Hypoactive Sexual Desire Disorder (HSDD) – but this diagnosis explicitly requires that "if a low desire is better explained by self-identification as asexual, then a diagnosis of [HSDD or arousal disorder] is not made" .
In the absence of the medical profession diagnosing asexuality as an illness, such a diagnosis should also be avoided by the general public.
No link to psychopathology
One study  found evidence that asexuality should not be classified as a dysfunction. There is no known link between asexuality and trauma, and no correlation between asexuality and psychopathology [3,4]. The lack of correlation suggests that asexuality in-and-of-itself is not a mental disorder, the underlying cause of the anxiety, or a result of the anxiety [3,4]. In fact, the higher rate of anxiety was documented only in the subset of asexuals that have had phobic experiences .
Although asexuals report a higher incidence of anxiety and mood disorders, "the available evidence seems to suggest that those feelings are a result of prejudice and discrimination against asexuals" .
On a deeper level, for asexuality to be a mental illness requires a strange conception of medicine in general. What is it that makes some conditions (e.g. tuberculosis) a disease and other conditions (e.g. blonde hair) not? The answer is our relationship the effect of the condition. If a condition doesn't cause someone distress, or make them a danger to others, and they don't want to change it, there is no reasonable conception of illness that can be applied to that condition.
When considering whether a condition requires treatment, the primary question we need to ask is: does it cause you distress, arising from inside the person? If someone has always felt asexual and it doesn't bother them, then there's no need to worry. Something can only be wrong if we decide we want it to be different.
- : Diagnostic and statistical manual of mental disorders : DSM-5 (5th ed.). American Psychiatric Association. ISBN 978-0-89042-555-8, p. 434 (for females) and p. 443 (for males). (You can find screenshots of these pages here.)
- : Brotto, Lori A.; Yule, Morag A.; Gorzalka, Boris B. (2015). Asexuality Is a Sexual Orientation, Not a Sexual Dysfunction. Psychology Today.
- : Brotto, Lori A.; Knudson, Gail; Inskip, Jess; Rhodes, Katherine; Erskine, Yvonne (11 December 2008). Asexuality: A Mixed-Methods Approach. Archives of Sexual Behavior. 39 (3): 599–618. doi:10.1007/s10508-008-9434-x.