incest probably never.
people would have said the same about gay marriage 100 years ago.
True, but there's definitely a difference.
Polygamy? It's just our monogamous culture that makes it such a taboo.
Incest? We've seen throughout history what happens when you engage in incest and reproduce. Genetic deformities. By the time we can overcome that, the question of whether there will even still be a need for physical sexual intercourse will be asked.
That's an argument against incest with intent to reproduce. Not against incest itself. Naturally or artificially sterile couples do not have the possibility of reproduction, nor do homosexuals. Even those who're not sterile can use contraceptives, and if you're not against abortion, use abortion as a last resort in case of failure of contraceptives.
And as said, what about those with serious genetic diseases? Do you also by virtue of this, and consistency, oppose their reproduction and believe it should also be restricted? The same question I ask for all those who followed with medical reasons, or "THINK of the children" responses.
I would make incest legal long before I would make polygamy legal. I think there is generally a natural aversion to incest, so If it were legal I doubt that their would be an increase in the number of siblings or parents and offspring wanting to have sex or get married. For the few people that really do want to have sex and/or get married to their parent, adult child, adult brother or adult sister, then I suppose letting them do so is only harming any potential offspring they might have, and isn't harming any current members in society. Polygamy on the other hand, is not something that people naturally have an aversion to, so legalizing it would most definitely lead to an increase in its practice. Polygamy, unlike incest, does impact other members of society in a negative way. Creating an imbalance between available females and available males has negative side effects. When available females are in short supply, there is an increased competition for females which increases violence and destructive behavior among males. Polygamy groups in Utah have more domestic violence, and young men in fringe Mormon groups, who fail to get married, or get kicked out of their community, often engage in destructive behavior (crime,alcoholism, drugs, ect.). Countries where polygamy is legal, or commonly practiced have more violence. Likewise creating a short supply of available males also has negative effects. Women in African American communities, where significant portions of the male population are in jail, engage in sexual activity at younger ages, are less likely to use condoms , and more likely to raise children as a single parent. The short supply of available males encourages females in these communities to be more promiscuous and engage in more risky sexual behavior in order to win over a male, men who are not in jail in such communities have less incentives to be committed to their partner sense other females are readily available. In short, legalizing polygamy would destabilize society.
A license to reproduce, and better automated security could deal with all of that. So we might have to wait a bit until it is viable. But you shouldn't restrict relations between consenting adults on the basis of what happens to the rest of society.