This social meaning consists of the web of shared understandings and expectations that have built up over centuries.
And he goes on later to reiterate this:
The “social meaning” of marriage — as I use the term here — consists of the understandings and expectations regarding marriage that almost all members of society share.
One worry about this immediately is the MacIntyrean question: Which society? This is a particularly gnarled question in a nation like the United States where legal issues are broken up into levels. There are federal issues with regard to marriage and state issues, and states differ considerably. There are more complicated issues, however, when we look at how Wedgwood glosses this issue of "shared understandings and expectations almost all members of society share":
So what exactly is this meaning? Since it consists of generally shared understandings and expectations, it can not include any controversial doctrines (such as the traditional Christian belief that marriage symbolizes the union of Christ and His church). It must consist in more mundane and less controversial assumptions about what married life is normally (though not always) like. These assumptions seem to include the following: normally, marriage involves sexual intimacy (which in heterosexual couples often leads to childbirth); it involves the couple’s cooperation in dealing with the domestic and economic necessities of life (including raising children if they have any); and it is entered into with a mutual long-term commitment to sustaining the relationship.
We get into the problem of the 'almost' here. Wedgwood claims it cannot include any "controversial" doctrines; taken flat this would imply that only understandings and expectations admitted by all members of society would be acceptable. But this would effectively make the whole notion of 'social meaning' useless; all it takes to have a controversy is one person who disagrees, and in a population like the United States, with well over 300 million people, for anything you could propose, there's someone somewhere who would disagree. So we really do have to take seriously Wedgwood's previous qualification of 'almost'. But -- and this is relevant to the question Wedgwood is considering, although it is not one of the more crucially important things -- how controversial does it have to be before we can no longer say that it is uncontroversial to "almost all"? Is this determined by poll and by some special threshold? Is it a pragmatic matter of how people actually act? Is it (and this is a question that would complicate Wedgwood's argument) a matter of what institutions and standards people actually make the effort to support?
What really strikes me about Wedgwood's list is that there is nothing distinctively marital about it in this day and age; if this is all there is to the social meaning of marriage, there is no fundamental difference between marriage and any other committed sexual relationship, even those that are explicitly anti-marital. This becomes something of an issue:
I propose that the crucial benefit is roughly this: by marrying, a couple can give a signal to their community that they wish their relationship to be viewed in the light of these generally shared assumptions about what married life is like. The rest of the community is not obligated to interpret the couple’s relationship in the light of these assumptions; but because marriage is such a familiar and generally understood institution, virtually the whole community will be able to understand the signal that the couple is sending.
Far be it from me to say that Wedgwood is old-fashioned and out of date, but this is an old-fashioned and out-of-date suggestion. Nobody marries for this reason because nobody needs to marry for this reason; and because of that, nobody expects marriage particularly to signal this because it's usually already been signaled. In this day and age people do this sort of thing by moving in together, not by marriage. Again, we come against the fact that in Wedgwood's account the social meaning of marriage, while relevant to marriage, has nothing particularly marital about it.
So what's missing? I think we get the answer when we go back and look at a point in Wedgwood's original description of shared meaning, the one he gave when describing the shared meaning argument against same-sex marriage, that fell out completely: "the web of shared understandings and expectations that have built up over centuries". People don't marry to signal that they are in a committed sexual relations involving a pooling of resources. You can do that easily in other ways. Nor does it make up a particularly salient part of the community's recognition of marriage. People marry to participate in the tradition, to join the club, so to speak; to do and be what married people have been through ages. And this is as relatively uncontroversial as everything else on the list. You do occasionally find same-sex partners who want to marry only for legal benefits, but usually people want to get married because they want to be married, to participate in the tradition, the institution, the practice, of marriage.
And it is here I think we hit an insuperable wall. For while it's relatively uncontroversial that marriage goes beyond other committed sexual relationships by being a traditional institution with benefits accumulated through that tradition and history by its contribution to larger society, it is highly controversial what the essential features of marriage as a traditional institution are. Opponents of same-sex marriage insist that once you get rid of the man-and-woman element of it then you what you have left is not really a way of participating in the tradition of marriage at all; it is at most superficially similar. Advocates of same-sex marriage insist that this is not true. And to resolve this dispute neither side can guarantee that anything to which it thinks legitimate to appeal will be shared by the other side. We are back at the question, "Which society?", because you can't talk about tradition without specifying which societies are to be taken into account. General features of marriage can be uncontroversial, part of this monotone general understanding, without providing sufficient specification to determine what specifics are necessary for those general features to be really -- and not merely apparently -- instantiated.
One of the fundamental problems with Wedgwood's talk of "social meaning" is that he appears to assume that there is one, univocal social meaning for everyone. I think this is manifestly false. There are broad similarities among very different conceptions. If you talk vaguely enough you can get something that widely diverse people will agree on, albeit for often very different reasons. And different societies and sub-society groups and communities will often have expectations and understandings that directly overlap with expectations and understandings of other communities. But what we find with social meaning is not one social meaning but a family of them, and when we try to sum them up we mostly get constructed types based on people's assessments of what's essential and incidental, or else such vague and patchwork happenstance-agreements that we can't tell what's actually essential and incidental. And it is, of course, entirely possible for people to make different judgment calls about what is essential and incidental in the things they agree on.
It's pretty clear that Wedgwood's post is not an argument for same-sex marriage. The argument actually depends on the premise that it is an injustice not to allow same-sex marriage, and this premise is not defended, so the argument presupposes that the case for same-sex marriage has been made, and made successfully. Rather, Wedgwood's argument is intended to be an argument against an argument against same-sex marriage; that is, it's intended to show that a particular argument against same-sex marriage, based on social meaning, fails. Wedgwood agrees that marriage does have a social meaning, but holds that this doesn't affect the case for same-sex marriage at all. I think his argument for this as written fails, for the reasons given above: (1) He is clearly missing some kind of key element in shared expectations and understandings of marriage and, I think, precisely the shared expectation and understanding that is causing the whole problem, tradition-participation; and (2) he doesn't do justice to the fact that you and I can have a very vague general-level agreement about some feature of marriage and still have an insoluble disagreement about what that general feature specifically requires in practice, and without dealing with this it's not really clear that he's given an adequate answer to the argument to which he is responding. But it is an interesting line of approach.