When marriage laws are amended to bring same-sex couples within their ambit, those couples become subject to the existing restrictions imposed by blood and family ties (consanguinity and affinity) which forbid people from marrying someone too close to them. So if a woman cannot marry her brother, uncle or nephew, neither can she marry her sister, aunt or niece. Simple, right? As far as it goes, yes. But not once we consider the rationale behind consanguinity restrictions and what the application of the "marriage equality" paradigm to them is likely to lead to.
Admittedly, the effect of consanguinity on marriage custom is not quite as straightforward as I have implied above.
In some ancient civilisations, consanguine marriage down to the level of incestuous marriage was obligatory among the nobility - the ultimate way of "keeping it in the family". In ancient Greece, half-sibling (same father) matches were not unheard of among the aristocracy. First-cousin matches were common among European royal and aristocratic families well into modern times. For the upper classes of yesteryear, inheritance considerations pertaining to the transmission of property and title understandably trumped all other considerations when it came to mate selection.
The first-cousins scenario raises the issue of the arbitrariness involved in judging how close is too close - in some states of the US you can marry your first cousin; in some, you can't. Cultural differences in attitude arise too: for instance, southern Asian societies, unlike most modern European societies, tend to endorse first-cousin marriages.
Variations notwithstanding, the avoidance of close (as defined by local standards) in-marrying has been a widespread social norm for centuries. To get to the nitty-gritty: sexual relations between people who are too closely related have generally been frowned upon by societies and legal systems throughout history, even inviting capital punishment in some. Incest remains a social and legal taboo - you simply don't go to bed with your close relatives. So, the gender issue aside, you cannot marry them either. But why not?
The consanguinity restrictions are underpinned by an excellent biological rationale - the avoidance of in-breeding. In-breeding brings out deleterious traits, mostly by "doubling up" the genes underlying them. It also results in the rapid spread of a bad gene throughout a small gene pool. (As animal breeders can tell you, in-breeding can be a good thing too - it can double up the good genes, or increase the prevalence of a desirable gene in your breeding stock, and improve a lineage - but you may have to weed out some of the undesirable genetic matter by eliminating affected stock on the way.)
Most closely in-marrying groups ended up paying the price for in-breeding. The Spanish Habsburg royal line ended after generations of in-breeding with the misshapen, dysfunctional and sterile Charles II who died at the age of 38 in 1700. Generations of high school biology students have learned about the spread of the haemophilia gene within European royal families in the 19th and early 20th centuries. Nature discourages in-breeding through built-in mechanisms in animals and plants to avoid, or at least minimise it; after all, sex evolved because of the adaptive potential brought about by the mixing of genetic material from different parents. Contentious but worthy of note in this context is the Westermarck Effect, which posits that people have a natural tendency to not be sexually attracted to relatives whom they grow up with, although it can be overridden by attitudes and values imbued by culture and upbringing.
It seems superfluous to draw the reader's attention to the fact that this rationale presupposes that marriage is between two people who potentially form a biologically fertile unit - a woman and a man. That some heterosexual couples are infertile through nature or choice is irrelevant to the application of the consanguinity restrictions - a sterile man wanting to marry his sterile sister or niece, or a closely-related elderly couple, are still prohibited from doing so; being a male-female unit, the potential is inherently there, however unlikely it is to materialise.
But the biological rationale for the consanguinity rules makes no sense in the context of two women or two men, as there simply can be no progeny produced between them, and hence there is no possibility of in-breeding. To apply the restrictions to homosexual couples is accordingly absurd. So why do we do it? I hear someone saying, "Because they have to be treated the same as hetero couples, dummy - marriage equality, remember?"
Wait a minute, though. There are two ways of treating the two kinds of couples equally - one is by applying the consanguinity rules to both, and the other is by applying them to neither.
I just gave my crystal ball a hard rub and this is what it reveals for the foreseeable future. First, a homo couple who are forbidden from marrying because of consanguinity will challenge the restriction, arguing that the rule ought not to apply to them because it is irrational to do so given that there are no grounds for it where a sexual liaison can not conceivably bear fruit. The case will probably have to go all the way to a Supreme Court or a supranational court such as the European Court of Human Rights, but in the end they will win their case and be allowed to marry.
Next, a hetero couple of the same degree of affinity as that homo couple will go to court arguing that to apply the consanguinity rules to them and not to the homo couple is discriminatory; if a man can marry his male relative, he can marry his female relative of the same degree of affinity. And they will win too, says my crystal ball.
Does this portend the demise of the consanguinity rules? It will certainly put pressure on them. If the couples involved are first cousins challenging the restriction in a jurisdiction that forbids first-cousin marriage, there is always the argument that the restriction at that level is not consistently applied across jurisdictions anyway, so setting aside the rule brings that jurisdiction into line with the rest. But what if they - or the next - couple to make an application are uncle/nephew or aunt/niece? And how long will it be before we are talking about a pair of true siblings (same father and mother)? Are we on the threshold of legitimising incest through marriage? We do seem to be gazing down a very slippery slope!
A quick Google search will reveal the emergence of an assertive "marriage equality for all, including blood relatives" movement. The current mood is conducive to that cause: we have, through legislation, seemingly convinced ourselves that the only thing that matters in relation to permission to marry is that people love each other (that is, have sex together) and that it is wrong to make any distinction between couples based on their genders because that's discrimination. Given those assumptions, the vignettes played out on my glass oracle become near certainties. We have backed ourselves into a corner.
The ensuing quandary - whether to impose on some couples a rule for whom it can not possibly have any consequence, or to simply dispense with the rules that prevent in-breeding - has come about as a result of the redefinition of marriage.
The very existence of consanguinity restrictions is a compelling reminder that marriage is, by definition, heterosexual. This assertion has nothing to do with homophobia as it does not denigrate people on the basis of their sexual preference - whom people go to bed with is their private business. But marriage as a social and legal institution has certain functions that can be fulfilled only in the context of opposite-sex liaisons.
The dilemma is a secondary absurdity arising from the primary absurdity of amending marriage law to encompass same-sex couples. Hopefully, a muddle such as the one my crystal ball has revealed will prompt legislators to revisit what is simply bad law.