Besides being confused about what the definition of asexual means, many people are also confused about it being a sexuality. I've read on AVEN and heard people describe to me in person that they feel that they "don't have a sexuality," and are therefore asexual. Moreover, this is often followed with "I'm nothing."
This is both inherently contradictory and a reflection of the way that sexuality is linked with personhood.
This should be straightforward. Asexuality = sexual orientation = sexuality. If you are asexual, your sexuality is defined by not being sexually attracted to anyone. It is a state of being. Your asexuality also shapes your interactions with other people as much as any sexuality does for anyone. You create relationships with people the way that you do because of your sexuality. You may do this differently than people with other kinds of sexualities, but your difference doesn't mean you don't have a sexuality.
Next, I should add that if you are asexual and describe yourself as "nothing," you're doing an injustice to yourself and are basically stating that you're not a person (whether this was your intention or not). A person is so much more than who they want to have sex with and how much sex they're having. Are people who aren't asexual "something," just because they feel sexual attraction? Perhaps you've been conditioned to believe that sexual attraction and being sexual are the most important parts of being human. Well, people who are not asexual are something, and you're something, too. You're an asexual, and you're many other things as well. You have hobbies, you have people in your life, you're talented at some things and not at others, you have likes and dislikes. I'm not trying to go on a "you're a special starfish" rant here, but basically everyone is "something" regardless of the kinds of relationships they want to form.
There may indeed be something inherently sexist about the assumption that all people are sexual, and therefore if you're not sexual you're not a person. Until a few decades a go it was accepted by Western medical practitioners (and in Western culture generally) that women could not have sex with each other, and that sex was something that only a man could have. It was also believed (again until recently, due to scientific inquiry) that women were not actually human, or if they were, they were subhuman men who didn't have enough heat in their bodies to create an external penis (the vagina was believed to be a penis that the body was too weak to push out, so to speak). Although in many cultures it is now accepted that women are human and can have sex without a penis, the notion that sex = male, only males = human and sex = human still has a lingering affect on how people are treated for being queer, not just asexual.