What we think of as traditional marriage is not universal. The default family arrangement in many cultures, modern as well as ancient, has been polygamy, not monogamy. The default mode of child-rearing is often communal, rather than two parents nurturing their biological children.
The point of this ideal is not that other relationships have no value, or that only nuclear families can rear children successfully. Rather, its 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.
So this is a conservative case against marriage rights is to recognize traditional marriage as social engineering and is worthy of support because of the pretty genealogy diagrams? Huh? I wonder if Mr. Douthat would support asexual reproduction because you can use Venn Diagrams for genealogy. I generally like Mr. Douthat’s column but this is really an awful argument in what is his worst column of the year.