olaf the tea drinker
I have serious doubts about the legitimacy of asexual being anything else then a hormone imbalance
Pairing behaviors observable in various species -- humans being the most notable (although we instinctively possess both monogamous and polygamous behaviors), but penguins and a few others share the nature -- are inherently sexual in nature. The desire to pair with a mate arises from an instinct whose sole purpose is explicitly in reproduction, and the mate one chooses is decided based on a set of unconscious criteria intended to identify who among a set of potential mates has the most desirable genetic material, in theory ensuring the genetic superiority of one's own progeny. The human is a species which reproduces sexually; therefore, all humans possess inherent sexual instincts.
Certain sexual behaviors arise in many species which naturally prevent reproduction -- homosexually; abstinence -- and it is thought that they are related to the abundance of resources in the environment. When fewer resources exist than are necessary to support a large extant population, that population may be observed to consume less food and water and engage less in sexual activity (an experiment into this was conducted using mice, in fact); conversely, should a small population encounter an extreme abundance of resources, they will instinctively engage in a state of heightened consumption and reproduction.
It is possible that asexuality is simply an instinctive response to human overpopulation, which would effectively disappear were most of the species to be destroyed. However, that it just my own supposition. I don't think that makes it illegitimate; I just understand that these things usually have an identifiable naturalistic cause.
As an afterthought, I suspect that, as most humans are at least somewhat prone to attraction to members of both sexes, while heterosexual behavior serves the obvious purpose of reproduction, considering the strengthening of emotional bonds between sexual partners and humanity's instinctively pack-type social structure, both heterosexual and homosexual behaviors serve the peripheral purpose of generating a greater sense of investment and community within one's tribe.
Again, though, that's just a guess.