I read here about human asexuality, and I cannot figure out if it's real or if people are just making stuff up. Have studies been done about this phenomenon and what explains it?
Yes, but until now only hypotheses on reasons for asexuality are available. Experiments on male gerbils show refused mating with female gerbils, indicating there are epigenetic, prenatal period factors causing diff. sexual orientation/behaviour:
For humans there has to be distinguished between an asexual lifestyle and sexual excitability/reduced libido, as some self-called asexuals are masturbating and obviously can feel orgasms, better called auto-sexual. Currently from a scientfic point of view there is no clear definition, but above mentioned properties would be obviously crucial. These articles sheds some light on classification of several defintions and genuine causes.
So phenomenological asexuality seems more to be a female "property", making a epigenetic more plausible than pure genetic causing of asexuality, as one would expect equal distribution.
Differences in human brain structure indicate, that asexuality is also not caused by purely psychological/social development reasons
From an evolutionary point of view one has to ask how likely a pure genetic heredity of a general asexual property is, as humans mainly bear single not several babies and the development help similar to the mentioned gerbil case cannot play a role.
Asexuality as a mammal phenomenon exists, but currently its not clear how much genetic, epigenetic and post-birth development factors actually contribute to this phenomenon. But current knowledge emphasize factors influencing fundamental brain structure rather than psychologigal/social reasons.
Special cases like genetic caused Asperger, Autism reducing will of physical closeness to other humans show set of difficulties defining and reasoning asexuality on humans.
It is estimated that ~1% of population is asexual (at least in the UK).
Source: http://articles.cnn.com/2004-10-14/tech/asexual.study_1_sexuality-new-study-new-scientist?_s=PM:TECH (discussing a study by Anthony Bogaert, a psychologist and human sexuality expert at Brock University in St. Catherines, Ontario)
However, it doesn't look like there are a lot - or any - studies on the topic which are more legit (e.g. not based on "have you had sex recently" surveys)
Recalling the teasing due to my 'eponymous' syndrome in 7th grade, there is such a thing as Turner Syndrome which leaves a person with a single X chromosome. Such a person would be asexual (although female), as the lack of the second X chromosome somehow inhibits development during puberty.