> Homosexuality is (like almost all complex traits[1]) influenced by thousands of genes in addition to environmental, social, cultural factors etc.

Homosexuality is also preserved across practically all social animals [1], and–I believe–absolutely all that form monogamous pairs. That isn't true for Down syndrome [2], which manifests much more like a disease.

[1] https://en.wikipedia.org/wiki/Homosexual_behavior_in_animals

[2] https://allthatsinteresting.com/animals-with-down-syndrome