If it's ok for one group it seems like it should be ok for the other. After all long as everyone is a consenting adult why can't people marry whoever they want? I suppose someone might say that with incest there is a chance of genetic defects, but this is a dangerous road to take. If the risk of genetic defects means people can't marry then there are a number of people in society who have known genetic defects that might be passed on to their children. Should they not be allowed to get married or have children?
The same-gender freedom to marry and the polygamous and consanguineous freedoms to marry are usually three separate things, but they all all part of full marriage equality. There's no good reason to deny an adult the right to marry and all consenting adults, and the US Supreme Court decision supports that. Although the case was dealing specifically with the limited monogamous same-gender freedom to marry, the text of the decision makes it clear that adults have a right to marry.
For incestuous mariage, okay but they shouldn't have right to make children, but adopt, because of the genetics problems children could have, and polygamy well that is weird but if everyone who is gonna be part of the "couple" is okay with it, why not ! It doesn't hurt anyone !
Although the idea might be disturbing, and you personally wouldn't do it, it does not give you the right to prevent others from having that freedom, although I feel incest marriages shouldn't have children because it could give the child defects, I'm not going to say it should be illegal because that's the same as saying woman over 35 cant have children because of the risk of down syndrome. People can love whoever they want and marry whoever they want.
Since the issue is about how people have sex (MSM,WSW etc.), then it should follow that any type of sex that is not in the majority should be legalized. Polygamy, incest, child marriage should and will eventually become legal thanks to the SCOTUS. Watch, wait and see-they will do it. :)
Law is not about right or wrong, Law is about what the majority of people find acceptable and consider right to maintain a civilised community.
The question of right or wrong is religious, so basically the answer is what ever you believe, and just because law allows gay marriage doesn't mean that Gay is right or wrong.
Theoretically, if Gay is ok, then the law shouldn't discriminate allowing 1 and not the others.
If a woman is allowed to marry another woman, the a man should also be allowed to marry another woman, So a polygamist Who has married one woman should be allowed to marry another woman without having to divorce his first wife first.
As for incest, people who commit this crime probably don't want to marry their sex partner anyway, so hey why not? If the state is handing out marriage licenses to gay couples, it is not fair to any other 2 people who want to marry, to deny them a marriage license.
The principle here is separation of church and state. The churches are teaching people that there is a right way to live your life and any other way is wrong. The church may have a right to suggest or recommend that other people should or should not do something, but any laws regarding marriage or sexuality is something they should have no authority over.
If man has adopted a 13 year old from Chechnya, and they love each other so they want to marry, I may object to that because the state would be giving him a license to commit incest and statutory rape. ,but, if she was really pretty and I was the man who adopted her, I would marry her, so who am I to judge?
Although I am unconvinced either way on whether or not polygamous and or incestuous marriage should be valid, homosexual marriage doensn't open the gate for them to be. They are totally different issues with different concerns that are unrelated to the concerns with gay marriage. Incestuous marriage could result in a greater risk of birth defects, and polygamy could result in the partners sharing the same person being dehumanized or brainwashed. These issues are not the same as the entirely benign gay marriage where the only concern of note was the inability to procreate.