One of the stock fallacious arguments against same sex-marriage is the slippery slope argument in which it is contended that allowing same sex-marriage will lead to allowing incestuous marriage. The mistake being made is, of course, that the link between the two is not actually made. Since the slippery slope fallacy is a fallacy, this is obviously a bad argument.
A non-fallacious argument that is also presented against same sex-marriage involves the contention that allowing same-sex marriage on the basis of a certain principle would require that, on the pain of inconsistency, we also accept incestuous marriage. This principle is typically some variant of the principle that a person should be able to marry any other person. Given that incestuous marriage is bad, this would seem to entail that we should not allow same-sex marriage.
My first standard reply to this argument is that if different-sex marriage does not require us to accept incestuous marriage, then neither does accepting same-sex marriage. But, if accepting same-sex marriage entails that we have to accept incestuous marriage, the same would also apply to different-sex marriage. That this is so is shown by the following argument. If same-sex marriage is based on the principle that a person should be allowed to marry the person they wish to marry, then it would seem that different-sex marriage is based on the principle that a person should be allowed to marry the person of the opposite sex they wish to marry. By analogy, if allowing a person to marry any person they want to marry allows incestuous marriage, then allowing a person to marry a member of the opposite sex would also allow incestuous marriage-albeit only to a member of the opposite sex. But, if the slide to incest can be stopped in the case of different-sex marriage, then the same stopping mechanism can be used in the case of same-sex marriage.
In the case of different-sex marriage, there is generally an injunction against people marrying close relatives. This same injunction would certainly seem to be applicable in the case of same-sex marriage. After all, there is nothing about accepting same-sex marriage that inherently requires accepting incestuous marriage.
One possible objection to my reply is that incestuous different-sex marriage is forbidden on the grounds that such relationships could produce children. More specifically, incestuous reproduction tends to be more likely to produce genetic defects which would provide a basis for a utilitarian moral argument against allowing incestuous marriage. Obviously, same-sex marriages have no possibility of producing children naturally. This would be a relevant difference between same-sex marriage and different-sex marriage. Thus, it could be claimed that while different-sex marriage can be defended from incestuous marriage on these grounds, the same can not be said for same-sex marriage. Once it is allowed, then it would be unprincipled to deny same-sex-incestuous marriage.
There are four obvious replies here.
First, if the only moral problem with incestuous marriage is the higher possibility of producing children with genetic defects, then incestuous same-sex marriage would not be morally problematic. Ironically, the relevant difference between the two that prevents denying same-sex-incestuous marriage would also make it morally acceptable.
Second, if a different-sex incestuous couple could not reproduce (due to natural or artificial sterility), then this principle would allow them to get married. After all, they are no more capable of producing children than a same-sex couple.
Third, if it could be shown that a different-sex incestuous couple would have the same chance of having healthy children as a non-incestuous couple, then this would allow them to get married. After all, they are no more likely to produce children with genetic defects than a non-incestuous couple.
Fourth, given that the principle is based on genetic defects being more likely than normal, it would follow that unrelated couples who are lkely to produce offspring with genetic defects should not be allowed to be married. After all, the principle is that couples who are likely to produce genetically defective offspring cannot be married. Thanks to advances in genetics, it is (or soon will be) possible (and affordable) to check the “genetic odds” for couples. As such, if incestuous marriage is wrong because of the higher possibility (whatever the level of unnacceptle risk might be) of genetic defects, then the union of unrelated people who have a higher possibiity of genetically defective children would also be wrong. This would seem to entail that if incestuous marriage should be illegal on these grounds, then so too should the union of unrelated people who have a similar chance of producing defective children.
In light of the above, the incest gambit against same-sex marriage would seem to fail. However, it also seems to follow that incestuous marriage would be acceptable in some cases.
Obviously enough, I have an emotional opposition to incest and believe that it should not be allowed. Of course, how I feel about it is no indication of its correctness or incorrectness. I do, of course, have argments against incest.
Many cases of incest involve a lack of consent, coercion or actual rape. Such cases often involve an older relative having sexual relations with a child. This sort of incest is clearly wrong and arguments for this are easy enough to provide-after all, one can make use of the usual arguments against coercion, child molestation and rape.
Where matters get rather more difficult is incest involving two consenting adults-be they of the same or different sexes. After all, the moral arguments that are based on a lack of consent no longer apply. Appealing to tradition will not work here-after all, that is a fallacy. The claim that it makes me uncomfortable or even sick would also not have any logical weight. As J.S. Mill argued, I have no right to prevent people from engaging in consenual activity just because I think it is offensive. What would be needed would be evidence of harm being done to others without their consent.
I have considered the idea that allowing incestuous marriage would be damaging to family relations. That is, the proper moral relations between relatives is such that incest would be harmful to the family as a whole. This is, obviously enough, analogous to the arguments made by those who oppose same-sex marriage. They argue that allowing same-sex marriage would be damaging to family relations because the proper moral relation between a married couple is such that same-sex marriage would damage to the family as a whole. As it stands, the evidence is that same-sex couples do not create such harm. Naturally, there is not much evidence involving incestuous marriages or relationships. However, if it could be shown that incestuous relationships between consenting adults were harmful, then they could thus be justly forbidden on utilitarian grounds. Naturally, the same would hold true of same-sex relationships.
Reflecting on incestuous marriage has, interestingly enough, given me some sympathy for people who have reflected on same-sex marriage and believe that there is something wrong about it. After all, I am against incestuous marriage and thinking of it makes me feel ill. However, I am at a loss for a truly compelling moral argument against it that would not also apply to non-related couples. My best argument, as I see it, is the harm argument. This is, as noted above, analogous to the harm argument used by opponents of same-sex marriage. The main difference is, of course, that the harm arguments presented by opponents of same sex-marriage have been shown to have premises that are not true. For example, claims about the alleged harms to children from having same-sex parents have been shown to be untrue. As such, I am not against same-sex marriage, but I am opposed to incestuous marriage-be it same or different sexes.