For Mc_Fuck yeah:
Neurological: The brain is wired in such a way as to desire certain thing... neurological needs. Humans are social animals and require stimulation from interaction with other beings. As an Autistic you may, like myself not require all that much. I do, however, desire romantic contact, not just all that much. That is neurological. The wiring in the brain is different.
Homonal, was misspelt. It was meant to be "hormonal". Any deficiencies in hormonal secretions from the endocrine gland can have major impacts on the neurological functionings of a human being. Think seratonin and depression. Sertonin is a hormone. Testosterone and oestrogen have been known to have major effects on the sex drive of a person. Too much, or too little, or a wierd balance of the two can have major effects on a persons sex drive.
Biological: Good ol' evolutionary imperative. This is actually a combination of the two above, but I seperate it out because it can include external factors such as vitamin defficiencies, nutrient deficiencies, (or overdoses of the same). Metal poisoning, chemical contamination. Any number of naturally occuring (and synthetic) compounds can cause disfunction in any number of systems of the human body. I am unsure which one would cause asexuality, but there is always the possibility.
I have never really researched into asexuality so I can not give a certain guess to the exact causes of it.
In response to Callaway:
Yes, surely you've heard of nymphomania?
In response to everyone else:
Thanks for the welcome, freedom of speech is certainly a wonderful thing. And I've already got to talk about my obsession and I have barely left the thread! This place is fantastic!