Asexuality isn't just 'not wanting to have sex'. Essentially it amounts to not having a sexual attraction to anyone.
What does that mean? You don't fancy anyone?
Do you have any idea how many people don't fancy anyone? How many can just take or leave sex?
It's not a thing. It's completely normal. So much so that until five minutes ago, it was often considered an innate part of the female psyche.
That, of course, was on the basis of sexism. But nonetheless, not having a physical reaction to another person is unremarkable.
Sex drive, and therefore arousal, is on a spectrum.
And until this determination to label every aspect of human behaviour, everyone knew that.