Related question: Can any consensual activity be wrong?

Disclaimer: This question has nothing to do with whether I agree or disagree with gay marriage! So don't overreact.

It seems to me that if gay marriage is legalized and allowed on the ground that "it is consensual between two person and involves no one else", then incest marriage should also be legalized and allowed on exactly the same ground, as long as precaution is taken not to have defective babies ( and even so, I am not too sure whether this is necessary). So in this sense we can say that gay marriage justifies incest marriage.

Anything wrong with my reasoning?

  • Incest may involve someone else after nine months.
    – user2953
    Commented Jan 1, 2016 at 5:45
  • 2
    Incest is illegal and a taboo for societal and cultural reasons, none biological. Cleopatra was the result of 27 generations of brother/sister marriage. If 2 thieves agree that it is ok to a consensual robbery of someone else does that make the robbery ok? I'm not arguing for or against gay marriage or incest, I just think 'consensual' does not make a valid argument for any moral or societal norm. Commented Jan 1, 2016 at 9:05
  • 1
    That sounds like your heterosexual bias talking there Keelan.
    – Neil Meyer
    Commented Jan 1, 2016 at 16:42
  • @NeilMeyer No, I had the same first thought, and am not heterosexual. Straight sex really is just different from gay sex in this way. And declaring scientific facts important to a potential child's quality of life to be irrelevant for the sake of political correctness is problematic.
    – user9166
    Commented Jan 2, 2016 at 2:15
  • 1
    This question is a valid question for the site and should not have been put on-hold. This smacks of trying to protect a "sacred cow" from criticism.
    – user18800
    Commented Jan 12, 2016 at 20:45

5 Answers 5


No gay marriage in itself does not justify incest marriage per se. Both are quite different. However reasons why gay marriage is justified may apply even to incest.

For example in Obergefell v. Hodges -

"The Court listed four distinct reasons why the fundamental right to marry applies to same-sex couples.

First, "the right to personal choice regarding marriage is inherent in the concept of individual autonomy."

Second, "the right to marry is fundamental because it supports a two-person union unlike any other in its importance to the committed individuals," a principle applying equally to same-sex couples.

Third, the fundamental right to marry "safeguards children and families and thus draws meaning from related rights of childrearing, procreation, and education"; as same-sex couples have children and families, they are deserving of this safeguard—though the right to marry in the United States has never been conditioned on procreation.

Fourth, and lastly, "marriage is a keystone of our social order," and "[t]here is no difference between same- and opposite-sex couples with respect to this principle"; consequently, preventing same-sex couples from marrying puts them at odds with society, denies them countless benefits of marriage, and introduces instability into their relationships for no justifiable reason.


These are also commonly given justifications by gay rights activists. Incest marriage satisfies all these criteria.

Although court opinion in itself is not a best philosophical foundation, I think but it is most popular one.

  • i think you misunderstand subjective language like "no difference". the point is that the legalisation of gay marriage changes marriage less like admitting incestuous marriage, than e.g. allowing divorce
    – user6917
    Commented Jan 2, 2016 at 8:30
  • I didn't exactly get what you are trying to say.
    – IsThatTrue
    Commented Jan 2, 2016 at 9:30
  • that there is no legal difference between straight and gay marriage besides the exclusion of laws which permit the latter. it's obviously a hot topic, but yeah
    – user6917
    Commented Jan 2, 2016 at 9:34
  • Now in some countries there ain't. But I was not arguing about legality, if I gave the impression. The response was just that reasons for alowing gay marriage could as well be applied to other forms of "marriage" wthout change or with only minor changes.
    – IsThatTrue
    Commented Jan 2, 2016 at 9:50
  • seems to me that marriage is ineliminably a legal term. ofc (morally) permitting incest would not shift the cultural landscape v much, but IMO it would change the meaning of what morality really means. ofc if you take for granted that homosexuality is (morally) closer to incest than heterosexuality, it would be v difficult to convince you otherwise. but one may ask why it would be necessary to convince such an incolutor.
    – user6917
    Commented Jan 2, 2016 at 9:55

Your reasoning is valid. However, the consensuality of the union is not the premise that defenders of same-sex marriage are basing their argument on.

Defenders of same sex marriage are working from the assumption that some gays and lesbians are born that way, and they cannot help or change the fact that they are attracted to people from the same sex.

This makes preventing same sex marriage while allowing opposite sex marriage a form of discrimination, since it involves denying a class of people a right based on something they were born with, while granting that right to another class of people. They argue that banning gay marriage is similar to preventing people from getting married based on their ethnicity or skin color.

Presumably incestuous or polygamous attraction is something that can happen to anybody regardless of their birth, and does not make those who have such feelings a distinct class of people, the way being gay does. Banning those forms of marriage do not constitute a form of discrimination, the way preventing same-sex or mixed race marriage does.

  • I know of no constitution in the world that defends the right to marry.
    – Neil Meyer
    Commented Jan 1, 2016 at 16:40
  • @NeilMeyer The Supreme Court disagrees with you. Both the Loving decision on interracial marriage and the recent ruling on gay marriage presume a right to choose one's partner unless there is a compelling interest to the contrary. This continues to be defended, e.g. nationallawjournal.com/id=1202734919108/…
    – user9166
    Commented Jan 2, 2016 at 3:20
  • 1
    @NeilMeyer: German constitution (Grundgesetz) article 6, sentence 1: "Marriage and family stand under eminent protection of the governmental order." (own translation)
    – Philip Klöcking
    Commented Jan 2, 2016 at 15:53

If that were the basis for allowing it, your logic would hold water. But it is not the basis for allowing it. And in fact (as my terrible, rambling answer to the given related question tries hard to clarify) "the obligation not to interfere in consensual acts where no one else is involved" is a meaningless argument, without a lot of extra analysis of the meaning of 'involved'.

Given a very literal sense of 'involves', consensual sex between two people who are married to other people involves no one else. If their spouses never found out and they could guarantee no harm would befall them due to this liason, then, this logic would say they should go ahead. And that is nonsense.

You need some standard for 'involves' that is broad enough to rule out this case. If you are married, your spouse must somehow be automatically involved in decisions you make to give consent to sex.

If you are swingers, any children should also be automatically 'involved' in such a decision. Perhaps not directly as participants in the negotiation, but via your obligations to them.

You and your spouse might adopt an open marriage. But you should not do so in a way that endangers your children's reliance upon you as a couple. For example, you should not mix partners in a way that might allow a paternity suit by a temporary partner to take children away from you as a couple who genuinely consider themselves yours, and depend upon that connection. Doing so disrespects your role as parents.

This is not even necessarily about sex, just about family identity. The same should be said of shady adoptions. If you adopt a child in a way that allows the biological parents to claim him back, this is exposing him and any real or potential siblings to unnecessary potential trauma. Planning for permanence in the parental bond is part of the deal. This obligation is only compromised when doing so is clearly in the best interest of the child, not when the parents or others choose to risk it for selfish reasons of their own.

Another part of the deal that traditionally comes with the family is that children should have a space safe from the pressures of sexuality and its related power issues. That place, in Western tradition, is presumed to be the extended family. So unless you make extraordinary plans to create that space elsewhere, you should forbid incest within the circle that is meant to constitute that safe space. Having, for instance, one set of siblings pair off romantically, breaks down the whole presumption that family provides a safe place for sexual innocence. So these people are involved, when you make decisions about marriages.

Gay marriage does not challenge the traditional safety that parents are obligated to extend to their children in the way incest does. So these are not parallel arguments at all. They involve very different standards as to who is 'involved'.


Gay marriage and incest are quite different, actually, and neither justifies the other in itself. However, the postulate that the State or the society has no power or function to intervene in consensual sexual activity does justify the legalization of incest, unless restrictions of application are imposed upon it. Such restrictions can be set either to justify only incest, only homosexuality, both, or neither. But unless such restrictions are applied, the same reasoning that allows homosexuality works on incest.

  • that is a point i too wanted to make. but as above i would suggest that the legal justification of gay marriage implicitly includes the legality etc. of homosexuality, as a part of it
    – user6917
    Commented Jan 2, 2016 at 9:37

It seems to me that if gay marriage is legalized and allowed on the ground that "it is consensual between two person and involves no one else", then incest marriage should also be legalized and allowed on exactly the same ground, as long as precaution is taken not to have defective babies ( and even so, I am not too sure whether this is necessary). So in this sense we can say that gay marriage justifies incest marriage.

Anything wrong with my reasoning?

What is wrong with the reasoning is that you blurred the moral claim for homosexuality and the legal claims on gay marriage.

i.e. homsexuality is morally "allowed on the ground that":

  • it is consensual between two persons without harm

and gay marriage is being legalised "on the ground that":

  • it is the union of two persons who can legally have sex

This should help you work out what is being claimed in the legal decision above.

  • I am not asking about comparing homosexuality and gay marriage , I am asking about the comparison between gay and incest marriage. Not too sure whether your answer is related to my question.
    – Graviton
    Commented Jan 2, 2016 at 9:56
  • @Graviton i tried to explain why the argument for gay marriage does not allow incest marriage. your reasoning above missed that homosexualty is both legal and morally permitted. this is implied in the discussion of gay marriage: and your reasoning is incomplete
    – user6917
    Commented Jan 2, 2016 at 10:04
  • Are you saying that legalizing gay marriage cannot be used to justify legalizing incest marriage because homosexual is morally sound but not incest? If yes, why don't you just say so in the answer?
    – Graviton
    Commented Jan 2, 2016 at 10:07
  • @Graviton in effect. sorry i thought you were IsThatTrue, so i was also responding to their answer
    – user6917
    Commented Jan 2, 2016 at 10:10

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