If sexuality is genetic, how did asexuality evolve?
Update:
See @Bryan Krause comment. Just because something exists (or equally, doesn't exist) does not mean that it was selected for or has any evolutionary relevance. Imputing selection to any particular state of affairs is usually bad reasoning without a lot of evidence.
I didn't do a good job of coming out and saying this in my answer.
My answer is best understood as answering the question: how could asexuality evolve under selection?
It does not at all address the question: DID asexuality evolve under selection (in humans)?
This second question is one for which the answer is probably &quot;no&quot;. Without more evidence, we cannot say whether asexuality has any evolutionary/selective relevance at all.
Original answer:
I would suggest looking more into the evolution of social animals for which large proportions of the population is non-reproductive (or sterile). There are many such examples, with insects as the obvious case:

Contrary to widespread belief based on heuristic arguments of genetic relatedness, non-reproductive workers can easily evolve in polyandrous species. The crucial quantity is the functional relationship between a colony’s reproductive rate and the fraction of non-reproductive workers present in that colony.

I would suggest looking more into that paper and citations therein. The question to answer is whether an individual's labor can be more important than their gonads to the continuation of their genes.
As suggested in the wikipedia article on homosexuality that you link to in your own comment, there is a positive effect on female fertility from having homosexual family members (&quot;kin selection&quot;). Granted, what causal mechanism is behind this is likely difficult to sort out.
It's quite plausible that asexuality behaves similarly to homosexuality in this regard. Those authors want it to be a genetic factor driving both sexuality and fertility. But to me it seems a bit more parsimonious to posit that it's good to have more warm adult bodies around to care for/support children.
From a practical perspective, if grandparents with non-functional gonads can help raise kids (i.e. increase effective fertility of kin), why not asexual relatives too?