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For me, the big question is evolutionary. Why does a trait like homosexuality, which is non-reproductive, persist in evolution when natural selection should have got rid of it?

This is a fallacy. Darwin himself even knew about Kin Selection.
Kin Selection says that, not only do individuals compete for selection, but families do! Darwin explained this in relation to producing cattle breeds.

The simple explanation is that in competitive environments, when there is redundant genes (eg. twins or siblings), the family can better guarantee that genes are passed on by using gays to not waste energy on procreating but rather to use energy to help raise their siblings and the young of their heterosexual siblings.

Furthermore, Group Selection is a theory that could account for altruism for people who are not your kin (otherwise what is the genetic advantage of caring for poor people?), as well as other phenomena like ants building bridges that can be passed on through memes

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