On the one side of the argument is the traditional appeal which privileges nature. That is, the biological and reproductive complementarity between a man and a woman is taken to be theologically and morally normative for Christian marriage--the icon of God.
On the other side of the argument, as I've argued it borrowing insights from Rowan Williams and Eugene Rogers, is the privileging of election. That is, the icon of Christian marriage, the image of God's love as expressed in YHWH's election and marriage of Israel, is how the marital union expresses the grace of election. In this view, election trumps nature, the same way grace trumped nature when God unnaturally "grafted" the Gentiles into the story of Israel. The same way election trumps the kinship bonds of the biological family. The same way grace is described as adoption along with reproduction. Overall, such a view of marriage includes marital covenants that fall outside the "natural" complementarity between a man and a woman.
Now, the point I want to make is that in the debates about marriage the battle is often fought between these competing visions. Is marriage defined by nature or election? Is the icon of God in marriage best captured by nature or election? Traditionalists argue for nature. Progressives argue for election.
But this is a false dichotomy, leading both camps to disparage, diminish and demean the icon of God as expressed in the alternative vision.
I don't think the grace of God as symbolically and sacramentally expressed and mediated in the covenant of marriage can be reduced to either grace or election. Both are involved. But far too often we diminish the icon of God held up by the other party.
Specifically, worrying as they do about heteronormativity, progressives will often seek to ignore, minimize or marginalize the biological complementarity and reproductive aspects of traditional marriage. Worries duly noted, I think this is a mistake. Clearly, God's command "be fruitful and multiply" mediates God's grace in the world, expresses the icon of God. Each one of us exists because of heterosexuality. Even gay couples wanting to have a family are dependent upon the reproductive grace of sex, their own of that of others. And the Christian response to the grace of reproductive sex, it seems to me, is a deep and lasting gratitude.
And yet, to swing to the other side, traditional Christians often ignore, minimize or marginalize the elective and adoptive aspects of marriage. In this, traditional Christians also err badly, demeaning the icon of God. There are things that nature can never, ever display about the grace and love of God. And the only marriages and families that can demonstrate this grace are those that do not, for a variety of reason, place biology and reproduction at the center. In fact, when biology and nature come to trump grace and election all sorts pernicious and dark theological dynamics are introduced into the Kingdom of God. These are same nature-based dynamics that Jesus's aggressively rejected throughout his ministry.
In short, both nature and election reflect vital, distinctive and complementary aspects of the image of God. Alone neither vision is complete, failing to incorporate critical theological aspects of God's covenantal love and fidelity as revealed and symbolically expressed in the biblical narrative.
A point here is that if you argue that the natural biological complementarity involved in heterosexual marriage cannot be rejected because it carries--biblically, theologically and sacramentally--the image of God then I'd like to say that I'm in perfect agreement. And the stronger you make that case the more I will agree with you. You cannot make that case strongly enough.
But where I will differ is if one goes further to say that the icon of God in marriage reduces to or is exclusively manifested in heterosexual marriage. I reject that reduction as a false dichotomy that leaves out critical biblical, theological and sacramental material related to how the grace of election expresses the love of God in a way that nature cannot.
In a similar way, I think Christian advocates of same-sex marriage, rightly worried about the oppressions of heteronormativity, are too quick to ignore or explain away how heterosexuality and procreation function as deep biblical symbols and, thus, as conduits of God's revelation of grace.
In sum, I don't think there is a choice here. It's a both/and rather than an either/or.
And all that to say nothing about how singles are icons of God in a way married couples--gay or straight--can never be.
That single person, sitting with her friends in the pews.
That gay couple, always with their kids in cute matching outfits.
That married couple, always running late to services because they have nine children, and she's pregnant with the tenth.
All these, and more.
All these, the Icons of God.