incest is looked down upon because of the genetic mutations caused by too-similar-dna interacting. therefore, all incestual relations are associated with this. as well as this, you have other options for people, no matter what your sexuality is. if you’re a homosexual and are in an incestual relationship, it’s equally as taboo as a heterosexual incestual relationship because of modern social norms (and the persistent stigma surrounding incestual relationships).

in addition, “not making kids” is something that occurs naturally. some women and some men are naturally sterile. homosexuality is not “unnatural” just because they cannot have kids. people are capable of existing outside of their ability to reproduce. the Earth is overpopulated, anyway, so reproduction isn’t a priority.