Last week, I posted a link to Jesse Bering discussing what we think we know about predicting sexual orientation. Again, I recommend that article, if the subject interests you.
Here's another worthwhile offering from Bering. In this post he discusses asexuality:
[S]ome scientists believe that there may be another sexual orientation in our species, one characterized by the absence of desire and no sexual interest in males or females, only a complete and lifelong lacuna of sexual attraction toward any human being (or non-human being). Such people are regarded as asexuals. Unlike bisexuals, who are attracted to both males and females, asexuals are equally indifferent to and uninterested in having sex with either gender. So imagine being a teenager waiting for your sexual identity to express itself, waiting patiently for some intoxicating spurt of lasciviousness to render you as dumbly carnal as your peers, and it just doesn’t happen. These individuals aren’t simply celibate, which is a lifestyle choice. Rather, sex to them is just so... boring.
But it's complicated:
For example, as discussion on the AVEN (Asexual Visibility and Education Network) website forums demonstrate, there is tremendous variation in the sexual inclinations of those who consider themselves asexual. Some masturbate; some don’t. Some are interested in nonsexual, romantic relationships (including cuddling and kissing but no genital contact), while others aren’t. Some consider themselves to be "hetero-asexual" (having a nonsexual aesthetic or romantic preference for those of the opposite sex), while others see themselves as "homo-" or "bi-asexuals." There’s even a matchmaking website for sexless love called Asexual Pals. Yet many asexuals are also perfectly willing to have sex if it satisfies their sexual partners; it’s not awkward or painful for them, but rather, like making toast or emptying the trash, they just don’t personally derive any pleasure from the act.
As always, if we wish to understand a given individual, we have to listen and ask questions. A behavior or lack of a behavior, a concern, a like or a dislike that appears similar across individuals can turn out to be quite different in character and origin. This is revealed when we discuss more deeply what a person thinks, feels and means by what they say. I've seen this demonstrated again and again in people who tell me they have no interest in sex or don't like sex.
It isn't difficult to imagine the possibility that there is such a thing as primary asexuality, a difference in the brain that simply rules out sexual interest and attraction, but as Bering explains, there is so much variation out there, that we really can't yet say much with confidence.