Absolutely NOT!!!
Oh good Lord, has it really come to this? Where any relationship roll of the dice now calls for legitimacy from the state by way of marriage? Oh come on.
"Government stay out of my bedroom"- Um, as far as I know, every single adult in this nation is free to have a consentual sexual/intimate/romantic relationship with pretty much any other adult they choose. The government doesn't stop it. So, have at it. Enjoy your brother, sister, aunt, mother, father, uncle. All day/night long for all I care. But, that isn't what is being proposed here. We are talking about state sanctioned marriage.
All of a sudden we should feel an obligation to sanction and legitimize every form of relationship with marriage? Why?
This is worlds apart from homosexual marriage, but that and the ewww factor tends to be what makes some people a wee bit leery to stamp a big fat NO on it. I guess it is the result of the slippery slope argument. As a society we do have the right to say "NO, this is deemed immoral and not beneficial to society in any way, shape or form to legitimize it with the benefit of marriage." We have the right, as a nation, to decide that some things are immoral...not for religious reasons (athiests have morals, right?)...but just flat out not morally beneficial to society. Are we really saying there is no moral line society is allowed to have in place? I mean, that really is the issue. Anything and everything must be legitimized and legally supported by society? I don't think so.
Homosexuals can make the argument that they were born gay and being unable to marry a same sex partner limits their rights. I know many disagree with that, but it cannot be proven either way, so it is a valid argument. How can you apply any of that to familial marriage? It really can't be. Homosexuals can say "I am not wired to have an intimate relationship with the opposite sex", but there is no evidence that anyone on this earth is only wired to marry within their immediate family.
"What valid reason beyond the ick factor is there to deny this type of marriage?" How about the fact that there is a reasonable amount of risk that family members could be influenced from infancy into this sort of relationship? Manipulation by an parent or older sibling during childhood could be an essential brainwashing that leads to this sort of relationship. And if you allow siblings, why not parent/child...as long as the child has reached adulthood? How can sibling marriages be allowed and not parent/child marriages?
Family members, whether it be parents or siblings can be considered an authority influence enough to skew the idea of real consent for me to think it is not beneficial to society to legalize and legitimize immediate familial marriages.
As far as the rights gained by marriage. Most of the rights are already pretty much there anyways. Making medical decisions, inheritance. Next of kin, how convenient is that? The only one I can think of is medical insurance. And honestly, I think medical insurance should get to the point where coverage is based on the amount of people you want to put on it in your household, where you would just pay accordingly by amount of people covered (whole other debate)
Sheesh, it blows me away that there are really people who think this sort of thing should be legitimized, that do not see the potential for abuse and coercion. Yes, this can all happen anyways, that cannot be stopped. But, I have no interest in seeing society stamp a big nod of approval on it by sanctioning it with marriage.
And if there are so few who would even want to do this (surely), then there really isn't a need for it anyways or a need to make it more beneficial to engage in such a relationship.
As always, JMHO. (btw, I know this is only a debate and not a law about to be written...just debating the idea of it)
Thank God children are still considered lacking the ability to consent. Is that next? I know NAMBLA is fighting it already.
I think you make a good point about the potential for coercion if children are raised together with the idea that they are potential sex partners/spouses, and especially if parents look at children that way. This is something I stressed about why I am undecided. I think it is a real concern, yet I'm just not sure that giving legal benefits to people who do want to marry siblings (who, if the psychological stuff mentioned in this thread is true, will be almost exclusively siblings who were raised alone) is going to encourage anyone to accept incest as a possibility for themselves who wasn't already interested.
Many of your other arguments, however I don't agree with. You ask why we feel the need to sanction
every relationshp with marriage. I wonder if sanctioning is really what we're doing (or what we should be doing) when we allow people to commit to a purely legal contract. If "sanction" was what was at issue, then wouldn't we have more restrictions on marriage? No more Vegas weddings. No more marriages for swingers. We nullify the marriages of couples in which there is physical abuse. I'm guessing most people don't sanction those relationships, but no one argues that such folks shouldn't be able to get married. Personally I have no need of the approval of the state or anyone else and I don't think of marriage as anything but a legal contract. If I wanted sanction, I could get that from a church without the legal contract. What I do need and want, though, are the over 1000 legal rights and benefits that come from marriage.
You bring up morality, but you don't say what is morally wrong with incest (apart from the coercion problem). It's very interesting actually. There have been some studies done in which people were asked about the moral status of various "eww" situations in which no one was harmed--adult sibling incestuous sex, the family cat gets run over and the family eats it for dinner, what if a man buys a frozen chicken and takes it home and has sex with it, etc. What they found was that most people were sure that these things were wrong, but they couldn't offer any good reason at all when interviewers talked to them. And yet, despite having no good reason at all, almost no one was willing to change their mind. Moral philosphers have concluded that what's happening in all of these cases is just "The Yuck Factor"--people have such a visceral knee jerk emotional/disgust reaction that they judge it to be obviously morally wrong, though they can't explain how there's any harm in it. (Of course, the coercion concern does show how there could be harm if as a society we started accepting or encouraging incestuous relationships. In the study the case was one in one time two adult siblings were alone camping and had sex once (and the female was infertile) and never did it again and neither was anything but happy afterwards.)
I'm also worried about how you distinguish gay marriage and incestuous marriages--that gay marriage ends up coming down to the argument of "I was born such that they could never love someone of the opposite sex." Well this is problematic for a couple of reasons. First gay people have different views about how their sexual orientation developed. No one thinks it was a choice (like, "do I want cheesecake or pie for dessert?") or anything that ridiculous. But, many people (I find women more than men) do not have some sense that it's just in their genes or that they always knew it since they were 3 or whatever. Second there are lots of people in same-sex relationships of whom it is not true that they can't love someone of the opposite-sex. I'm bisexual. It's possible I could love someone of the opposite-sex, but thus far I never have. I do love someone of the same-sex though and we want the same legal benefits straight people get. I don't think whether I should get those benefits is a matter of whether I
could love someone of the opposite sex.
On the issue of the rights of marriage, you mention that close blood relatives would already have certain rights that non-related unmarried couples wouldn't have. This is true to some extent (though, if there are other living blood relatives, wouldn't they have just as much claim as the sibling-partner?), but there are a ton of rights that blood relatives don't have that married people do (in addition to medical insurance):
* Assumption of spouse's pension
* Automatic inheritance
* Automatic housing lease transfer
* Bereavement leave
* Burial/remains determination
* Child custody
* Crime victim recovery benefits
* Divorce protection
* Domestic violence protections
* Joint adoption and foster care
* Leave from work to care for a sick partner under family medical leave laws.
* Leave from work when a spouse gives birth or adopts under parental leave laws.
* Legal co-parenting (Insurance, School Records, Medical Decisions, etc)
* Medical decisions on behalf of spouse
* Visitation of spouse's children in the event of separation
* Wrongful death settlements
* The right to become a legal parent to a partner's child.
Personally I feel that the only reason I have heard anyone give so far that could make me think it
might be okay to deny a couple these rights because they are related by blood is the concern about coercion.
What I would really prefer, however, is that we just opened up the legal benefits in a way that doesn't imply that people who make that contract with one another are romantically in love with one another or have sex with one another. Take a case where two sister's husbands die young and the sisters decide to move in together to raise their children together. They live together for 15 years while they raise their kids, but they enjoy none of the benefits of the legal contract they had with their husbands. Why shouldn't the state be giving people in those situations legal benefits that protect them and make it easier to raise their children together?
. Things could also be very sticky regarding children--adoption, custody arrangments...how do you do that with 30 spouses? What about jointly following taxes--and we thought the current IRS forms are confusing! 
