If I were a betting man and had to wager what the most offered reason to officially recognize same-sex marriage is that any consenting adults should have the liberty to enter a marriage contract. To me, consenting adults is a bit too all-encompassing, morally speaking.
Consenting adults could include parents and their own adult (and teenage) children and siblings. Given that ages of consent vary from 13-18 in the United States, some children could legally consent to incestuous sexual relationships with adults as well as other children.
So the question is this: If consent is a prime factor in whether marriage laws should be altered, or acceptance or endorsement of sexual relationships should be granted, are laws banning incest immoral — should they be repealed? If we follow the rationale to its logical conclusion it seems that yes, they should be repealed.
If we as a society ought to accept and champion same-sex sexual relationships because they are between individuals who can legally consent to such relationships, then doesn’t it follow that incestuous relationships should enjoy the same stigma-free endorsements if we’re being intellectually honest? If one is to argue in favor of same-sex marriage, but against brothers marrying their sisters (or brothers for that matter), or fathers marrying their teen daughters et al, how do you do so without inconsistency?
Sifting Reality 
If same sex marriage is legalized, can two brothers marry? Can two sisters marry? Can a man marry his son? Incest laws are about protecting from genetic defects in children, but since these unions can’t produce, does it matter about their relationship?
If you look at the context of the current SSM debate, the only logically consistent answer is no. Since the reason du jour is that it’s wrong to prevent people who love each other from marrying, any limits are purely arbitrary. I’m actually surprised how few on the pro SSM side are willing to admit this.
According to some people I’ve asked, we should allow father/son, mother/daughter, brothers, sisters. Anything goes. Their definition (any) necessarily includes those relationships. More precisely, their willingness to throw out restrictions on marriage puts them in a corner from which they have to allow anything in order to get what they want.
Even more than two consenting adults should be allowed to marry, they say (they must). The key point there being that if there’s no reason “any TWO” can’t marry, why not allow more? As long as we’re redefining, why not jack with the number of people in the definition?
There are good reasons not to allow “any” two consenting adults to marry. Close family, specifically. But, there is only one reason to prefer and allow TWO people to marry. If the COUPLE is comprised of the type of people who can make babies (one man and one woman), it behooves society to have a way to lay out their rights and responsibilities.
I keep harping on the whole procreation aspect of marriage. Don’t we need two people of the opposite sex to get married? Isn’t that reasonable? And isn’t it because of the likelihood that they will produce children?
Male/female marriage is less a matter of their desire to marry as people in love than it is a matter of their responsibility to marry as POTENTIAL parents.
We have to remove the procreation aspect of marriage (and they do) to imagine that any two people fit the definition.
Yes, the logical conclusion is that truly ANY two (or more) should be allowed to marry. The next step is one that we should all fear. If there is no reason any configuration of people shouldn’t marry, what is the reason any male female couple should?
C2C
Yeah there are some commenters here who have said they wouldn’t mind lifting restrictions on incest. However I can’t help but to think its merely to defend turf. I mean, its possible that they are so morally broken that they don’t see a problem with it. But I have the feeling they know the same arguments used to support homosexuality and same sex marriage can be used for any sexual pairing.
We can continue to give our best philosophical and apologetic discussions about this and the devil will still produce counterfeit equivalents. Not only are the definitions of “man” and “woman” up for debate but “procreate” is up for debate. It’s likely that lesbian and gay groups will simply provide egg and sperm donations among each other and either willing lesbian or straight women will carry the artificially inseminated child to term. The enemy’s plan is unraveling. All of the fertilization discoveries (in-vitro, surrogacy, egg freezing) are vital to the same sex marriage.
“It’s likely that lesbian and gay groups will simply provide egg and sperm donations among each other and either willing lesbian or straight women will carry the artificially inseminated child to term.”
This is already happening. Sadly, this has resulted in desperately poor women in India (I don’t know why it’s India in particular) being paid to be surrogate mothers. Without access to decent food or proper prenatal care, many have died.
The problem with reducing marriage to being about “love” is that at that point, anything goes, so long as the people involved claim to love each other. Consent is little different. There is that professor in the US who is in a sexual relationship with his adult daughter, and fighting to have it legalized on the basis of consent. “Minor attracted persons” are convinced their victims are willing partners. Even “zoophiles” believe their animals are willing and can give consent. People always get very upset when I bring these examples up, but the reality is that recognition of SSM in Canada has given legal ammuniton to those who are fighting to have their “alternative sexuality” legally recognised and accepted.
Kunoichi is right. Of course others would say “me too, for the same reasons gay marriage is allowed”. The reasons given for gay marriage work for “any” two people.
This acceptance of marriage for anyone who wants it ignores that there are certain people who need it, and who we need to have it. But that is not to say that they are somehow more worthy of marriage. Just that they are in need of it.
It’s not that they are who they are, or love who they love. It’s that they do what they do. Who needs liability insurance? People who drive cars. Who needs homeowners insurance? People who own homes.
Who NEEDS marriage? People who engage in activities that MAY result in a situation that would necessitate a promise to stay together.
Golfing won’t result in a situation that would make society prefer that I promise to stay with my golf buddy forever. Neither do walks in the park or gazing into each other’s eyes. Heck, not even sex does… UNLESS it’s between one man and one woman.
Marriage is baby insurance.