Just a little more to add. I was thinking further about the inconsistency, within my argument, of allowing infertile and older couples to marry. I realised that this issue is a red herring, and I want to try to explain why.
First, that the existence of marriage in the form we have it is primarily to provide a context for reproduction I think can readily be seen from the structure that it has, as well as considering its probable evolutionary history, as I already explored in my post #108, and won't waste space by repeating here.
Second, that there are some relationships currently permitted within the scope of marriage that do not lead to reproduction is not particularly an embarrassment to this case, and certainly does not give ground to expand the scope of marriage to other relationships that also don't lead to reproduction, for the reasons I shall explore below.
1) Traditional Marriage has never been codified, but is handed down to us in common law. This is the case with many ancient traditions that have passed into our modern law, and the English-speaking world operates with a common law system that accepts this (and the discussion in this thread has been primarily about US law). It does not mean that the system that was handed down has to be entirely consistent with the definition that may be implicit from its form. It is only if we are making a change to the system that we need to work through a self-consistent system, within our current jurisdictions. If we decided to change the system to exclude some forms of marriage that were previously permitted, then we need to explain carefully why - so a case could perhaps be made for excluding infertile and elderly couples from marriage on the basis of the implied common law definition of marriage. Alternatively, we could make a case for including other relationships that are currently excluded, by changing the definition of marriage. But maintaining the status quo does not need to be defended within the common law system; it is changes to the law that need to be explained and made self-consistent.
2) The potentially inconsistent relationships that are currently included within marriage differ from the ideal as a matter of degree rather than being different in kind. This is not the case with the relationships that SSM advocates are seeking to bring in.
To a first approximation, what is required to make a baby? Answer: a sexually mature man and woman having sex. Therefore, to a first approximation, the people that should be included within the remit of marriage are any sexually mature man with any sexually mature woman. That is pretty much the boundaries for marriage, with a few exclusions that can be justified in statute law (close relatives, mentally incompetent people, children under the age of 16/18 depending on jurisdiction).
Infertility in most cases is a matter of probability, not of an absolute. It is not so much that it is completely impossible for the couple to have a baby, just that it is very unlikely. In some cases, injury or surgical procedure may make that probability actually zero. However, where should the cut-off point be? The couple fit the basic entry requirement of being a sexually mature man and woman. Should they be excluded if their chance of having a baby is less than 50%? 25%? 10%? 1%? 0.01%? Wherever you draw the line, it is rather arbitrary. Also, medical advances may mean that these probabilities change over time.
Similarly, with elderly couples, where should the cut-off be? Should we set an age limit? Or should we set a time period after the woman's last period? If an age limit, that ignores the fact that the menopause can come at quite different ages in different women. If a time period, that depends on all women keeping a record of this information, and being truthful about it. In both cases, it also ignores the fact that medical technology is pushing back the boundaries of when people can have children. And any cut-off point would again be arbitrary.
Also, with couples who don't intend to have children, this is also a matter of degree. What if a couple is 90% sure they want children - is that good enough? What if they are 50-50? What if they are 90% sure they don't want children? What if they start out 100% sure they want children but later change their minds? What if they start out 100% sure they don't want children but later change their minds? Any cut-off point will be arbitrary.
However, all these couples fulfill the basic eligibility requirements of being a sexually mature man and woman. There could be grounds for amending the law to exclude some of them, but there is no need to do so. The basic definition of marriage is not affected by their inclusion, since we understand that babies come from a sexually mature man and woman having sex. The difference between these couples and those who will actually have babies within their marriages is one of degree not of kind.
However, permitting two men or two women to marry is a difference of kind, not of degree. These couples do not fulfill the most basic eligibility requirement for being able to produce a baby. The definition of marriage will have to be changed if their relationships are to be recognised as marriage. If the definition is to be changed, the form of marriage should also be altered in order to reflect and be consistent with the new definition.
In a secular society, there is no absolute bar to making such a change. However, enacting a change by statute requires the issue to be thought through, and a consistent definition to be used, considering all the relationships that should be included under the new definition, and creating a form of marriage that is consistent with the definition. The current form of marriage is not suited for a definition of marriage that is not primarily about reproduction. In a democracy, the people should also be fully informed about the nature and implication of the changes, which profoundly affect society's structure as well as the way we think about ourselves, and asked to approve those changes.
The exact form in which that approval will be given will vary from country to country and is a constitutional matter. However, in any democracy the people should be made aware of the changes in definition that are occurring. There are at least two major changes in definition that are being made, as I outlined previously. One is that marriage is being defined not about providing a context for the rearing of children but about the love between the two (or more) partners involved. The second is that it is no longer being regarded as normative that a child should be raised by their two biological parents, but can be raised just as well by any two (or other number) of competent adults. These points should be made very clear and careful public consultation should be undertaken to ensure that the public is aware of the nature of the changes and approves of them. As far as I have seen, such an open and frank consultation has not taken place in any of the countries that have been considering introducing SSM. Consequently, the laws that have been passed or are being proposed to be passed are messy and inconsistent and open to future challenge in the courts.
Roonwit