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I think that one of the reasons that mainstream Christianity has done such a poor job of defending marriage is that this is an area of fundamental theological weakness - such that marriage does not have a sufficiently secure or central position in the mainstream Christian mode of life to enable it to survive the kind of determined secular-hedonic onslaught of the past century or so.
The root of this weakness is that mainstream Christian marriage is perceived as a merely this-worldly, hence temporary and expedient, institution.
By contrast, the Restored Gospel of the Mormon (LDS) church has been exemplary in its defence of marriage, as well as family; and this is substantially a consequence of its regarding marriage as (potentially) eternal.
Thus eternal marriage is not a defence of 'traditional' marriage - because traditional marriage is a much weaker thing than eternal marriage - the Mormon doctrine of marriage is radical, not traditional.
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From Adam S Miller Rube Goldberg Machines: essays in Mormon theology (2012)
Traditionally, marriage is not eternal. Rather the traditional meanings of marriage can be broken into two segments, both of which are rooted in finite interests:
1. Marriage as a hub of economic exchange and social productions, and
2. Marriage as an expression of preference in the pursuit of personal satisfaction.
These traditional meanings are not bad in themselves, but they are certainly not eternal...
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...The difference is this. In [mainstream Christianity] eternal life is singular. In the context of eternal marriage, eternity splits and pluralizes eternal life into eternal lives.
[Eternal] marriage transforms human sexuality from a merely biological difference into a truth that is both spiritual and eternal...
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Human sexuality is not reducible to biology; rather, human sexuality is irremediably grounded in the symbolic, spiritual dimension of the 'word'...
Gender, as the Proclamation on the Family ^ reminds us, is 'an essential characteristic of individual premortal, mortal, and eternal identity and purpose', and, as such, it is fundamentally a spiritual, rather than a biological, distinction...
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...Sexuation, in order to be thought as a truth, must be more radically rooted in two discrete relations to... symbolic order.
In relation to the symbolic, there is both a masculine position and a feminine position and these positions are incommensurable...
the feminine mode of grasping the symbolic, the feminine way of knowing the world, is fundamentally different from the masculine position, and vice versa...
Eternal marriage, rather than being the mutual satisfaction of sexuated interests and preferences, is an interruption or a calling into question of these preferences by the incommensurable logic of the other sexual position.
Love is an experience of the nonrelation of sexual difference. It is an exposure to the gap in being human that is human sexuality.
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In sum, for Mormon theology, our souls are, and always have been, and always will be, gendered: to be a man or a woman is part of each person's essence.
Eternal marriage has sexual identity as a primary and irreducible metaphysical reality (or assumption) - which makes eternal marriage between man and woman the most profound unit possible (i.e. the ultimate unit is a dyad).
And from this eternal perspective flows, as consequence, the essential or ideal nature of marriage in this mortal world.
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^ The Family: a proclamation to the World. 1995.
http://www.lds.org/topics/family-proclamation
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