The assertion of a belief, whether Christian or postmodern (pick your favorite label or eponym), is not an argument. That's all Douthat has done. Consider this passage:
The point of this ideal [of heterosexual marriage] 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.
The point I am making is this piece is mere assertion, and yet speaks to the stupidity of opinion pieces in popular media. Douthat offers no independent reason to convince us that the ideal of heterosexual marriage offers something distinctive. It is just the assertion that marriage ought to have this privileged status. This is what Corvino should have said about this op-ed piece.
Now, philosophers do this all the time -- they build up the background assumptions of an argument if they are not explicitly stated -- to the point that what they criticize is the best version conceivable. If that best version fails for some obviously flawed reasons, then we have a right to reject it and its lesser forms. As such, again, Corvino should be commended because I would not have the patience to be a philosopher with someone as moronic as Douthat.