Jesse Bering in Scientific American:
At least, that’s what most people tend to think. But actually, some scientists believe that there may be a fourth 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 bolus 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.