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If sexuality is genetic, then it stands to reason that evolution would strongly favor people who have children. In other words, heterosexuality & bisexuality should be favored, while homosexuality and asexuality should be disfavored and in fact eliminated by evolution. Why hasn't that happened?

I can find plenty of resources via Google for the evolution of homosexuality, such as this one, but not for the evolution of asexuality. Asexuality seems to pose a bigger challenge as well, since the lack of sexual attraction entirely should make it even less likely for asexual people to have children. Some of the explanations for the evolution of homosexuality can conceivably apply to asexuality too (such as "the genes for asexuality do other things"); still, I would like an authoritative paper/reference if there is one.

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    $\begingroup$ can you point to a reference that suggests that sexuality is in fact substantially genetic? i'm not disputing that you can find some signature of heritability, but for this variation to be selected upon, there should likely be a fairly strong genetic signal. furthermore, can you demonstrate that asexual/homosexual people have fewer children? this is less obvious than you seem to think it is, and it is also a necessary prerequisite $\endgroup$ – Maximilian Press Nov 9 '20 at 5:08
  • $\begingroup$ @MaximilianPress there's this that addresses both points for homosexual people. I have no sources for asexual people - it just seems natural, given what applies for homosexuals. $\endgroup$ – Allure Nov 9 '20 at 5:31
  • $\begingroup$ If 1% of a population is asexual, but can have children when they want to, or when a well positioned male wants to, perhaps that has a negligible or a positive effect on group success and evolution. external-preview.redd.it/… For some reason, I think that Freddy Mercury was on the far right of that graph. $\endgroup$ – DeltaEnfieldWaid Nov 9 '20 at 9:14
  • $\begingroup$ @aliential I don't understand - homosexual people can also have children if/when they want to, but they still have fewer children than the rest of the population. Presumably the same would also apply for asexual people. $\endgroup$ – Allure Nov 9 '20 at 10:17
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    $\begingroup$ Does this answer your question? Why do some bad traits evolve, and good ones don't? $\endgroup$ – tyersome Dec 20 '20 at 1:18
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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 "no". 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 ("kin selection"). 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?

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    $\begingroup$ There is also biology.stackexchange.com/questions/35532/… Not to say there is anything wrong about asexuality, but to say that the existence of a trait does not mean that the trait was selected for, and similarly, just because a trait does not exist does not mean that the trait was selected against, even if the trait is entirely genetic. $\endgroup$ – Bryan Krause Nov 9 '20 at 21:08
  • $\begingroup$ @BryanKrause that is an excellent point that I did not explicitly address. I'll edit. $\endgroup$ – Maximilian Press Nov 9 '20 at 21:59

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