"Interracial" marriage vs "same sex" marriage

[A Black Falcon]

Yeah, those are not like homosexuality. Incest has the problem of power relationships that you mention (the issue of consent in the all-too-common abuse scenarios incest often centers around), first, and second, has a higher chance of genetic problems if there are any children -- there's a reason why incest is one of the oldest taboos, dating back to the earliest days of humanity. The Westermarck Effect, as it's called (that children who grow up together as siblings from a young age don't see eachother as sexual partners), exists to ward off those genetic problems as much as it does to improve human societies.

As for polygamy, I think a relationship with one person of one gender and several of the other is fairly obviously sexist. There's no way to have a relationship like that and have it be equal! You can very much see this by looking at polygamous Mormons in the US, yes.