Ross Douthat explains that opposition to gay marriage is about trying to uphold a certain kind of ideal of marriage:
This ideal holds up the commitment to lifelong fidelity and support by two sexually different human beings — a commitment that involves the mutual surrender, arguably, of their reproductive self-interest — as a uniquely admirable kind of relationship. It holds up the domestic life that can be created only by such unions, in which children grow up in intimate contact with both of their biological parents, as a uniquely admirable approach to child-rearing. And recognizing the difficulty of achieving these goals, it surrounds wedlock with a distinctive set of rituals, sanctions and taboos.
The point of this ideal is not that other relationships have no value, or that only nuclear families can rear children successfully. Rather, it’s that lifelong heterosexual monogamy at its best can offer something distinctive and remarkable — a microcosm of civilization, and an organic connection between human generations — that makes it worthy of distinctive recognition and support.
I think one could dispute that, but let’s grant it. The natural thing to observe is that very little of our current legal architecture of marriage has much to do with this. Actual marriages in 21st century America aren’t required to be lifelong or monogamous. Douthat concedes as much:
Or at least, it was the Western understanding. Lately, it has come to co-exist with a less idealistic, more accommodating approach, defined by no-fault divorce, frequent out-of-wedlock births, and serial monogamy.
So at this point we’re upholding an ideal of lifelong heterosexual monogamy by legally requiring the heterosexual part, but not the lifelong or monogamous part. The unfairness of such a standard seems both obvious and overwhelming.
And the solution seems to me to be fairly clear—a separation of religious and quasi-religious ideals of marriage from the civil/legal aspects of marriage. You should have a defined legal state, that could be called “marriage” or “civil union” or “civil marriage” or whatever else we want that’s recognized by the state on a non-discriminatory basis. And then religious groups can also have whatever kind of ceremonies with whatever attendant status they like. If the Catholic Church doesn’t want to perform marriages for gay couples or allow divorced people to remarry, good for them.
But as Douthat’s piece makes clear, the status quo is really a cop out. Instead of holding heterosexuals up to a rigorous standard of conduct—no divorce, harsh & unforgiving attitude toward infidelity—we’re going to discriminate against the gay and lesbian minority and then congratulate ourselves on what a good job we’re doing of upholding our ideals.