"Do you think incest and polygamy will be legalized?" - Masterpost

I came across a poll on the subject of the future of full marriage equality:

I was watching a TV show recently and surprisingly, three characters (two men and a woman) got together in a sexual AND romantic relationship - as in, it wasn’t just a kink, they were serious about it, they even made their relationship official to their family/friends. And it got me thinking, do you guys think polygamy will ever be legalized? And incest? And should they be, and if not, why? Will our society ever see it as acceptable/normal like with gay relationships?

I thought it would be helpful to create one post where I link to everything:

• My initial response. I debunk the idea that no-one cares enough, argue against the usual eugenic argument, and also against the annoying argument I see all the time that amounts to, “Hah! All you can get is your mom?!” I also point out that polygamy/polyamorous marriage isn’t inherently sexist, because it includes polyandry and polygynandry. Women still have to consent.
• Eugenics is not a good argument, but it’s the only one that sounds kind of scientific and reasonable in people’s heads, so they always fall back on it. It doesn’t matter if it has strange and disturbing logical implications, apparently. Then I have to explain the difference between consensual sex and consensual cannibalism…
• I like this one. He provided me with an actual study for his statistic. I point out that it’s not as bad as he originally said, and either way the study itself points out that its findings are almost useless, because of sample bias.
• I have to further explain why eugenic arguments are logically dangerous, and how even by the standards of eugenics, laws banning consanguineous sex make zero sense.
• I lay down my arguments, once and for all, for why it makes sense and is just to have age-of-consent laws, but not laws against consanguineous sex. Let this be the end of it.
• I explain to someone why people are polyamorous, and how it can work successfully. I also point out an assumption in his own thinking about jealousy in monogamy.
• My closing statements.