The article argues for what the authors call the Conjugal View of marriage:
Marriage is the union of a man and a woman who make a permanent and exclusive commitment to each other of the type that is naturally (inherently) fulfilled by bearing and rearing children together. The spouses seal (consummate) and renew their union by conjugal acts—acts that constitute the behavioral part of the process of reproduction, thus uniting them as a reproductive unit. Marriage is valuable in itself, but its inherent orientation to the bearing and rearing of children contributes to its distinctive structure, including norms of monogamy and fidelity. This link to the welfare of children also helps explain why marriage is important to the common good and why the state should recognize and regulate it.
The article suggests two routes to the anti-gay marriage conclusion: (1) that the Conjugal View is a metaphysical fact, which ought to be reflected and enshrined in law; and in any case (2) it has better consequences than the alternatives. I'll address each of these in turn.
The Metaphysics of Marriage
Methodologically speaking, I find the "metaphysics first" approach to public policy rather bizarre. For example, when instituting an intellectual property regime, the core question is not "what is intellectual property?" (as if there were some pre-legal fact of the matter), but something more like, what values are at stake here and what policies/laws would best serve these values?
The authors argue that revisionists must accept the primacy of their metaphysical question, for "[o]therwise, how could the law get marriage wrong?" (p.250) The obvious answer is that some legal regimes may be better or worse at realizing relevant values. But anyway, putting the word 'marriage' aside, we can interpret the first part of the paper as arguing that there's something distinctively valuable about the kind of relationship described by the Conjugal View (a value not shared by any other kind of relationship). So let's consider that claim.
Their core argument appears on pp.253-4:
Marriage is distinguished from every other form of friendship inasmuch as it is comprehensive... [B]ecause our bodies are truly aspects of us as persons, any union of two people that did not involve organic bodily union would not be comprehensive—it would leave out an important part of each person’s being...
Our organs—our heart and stomach, for example—are parts of one body because they are coordinated, along with other parts, for a common biological purpose of the whole: our biological life. It follows that for two individuals to unite organically, and thus bodily, their bodies must be coordinated for some biological purpose of the whole.
That sort of union is impossible in relation to functions such as digestion and circulation, for which the human individual is by nature sufficient. But individual adults are naturally incomplete with respect to one biological function: sexual reproduction.
Now, I'm happy to grant the claim that there's something distinctively valuable about comprehensive relationships, in some sense of that term. But it is just bizarre to interpret 'comprehensive' here to require this technical notion of 'organic bodily union'. There doesn't seem anything normatively significant about (perhaps unintentionally) performing a "biological function" together. Why think that biological functions matter intrinsically?
(To bring out the absurdity, suppose it turned out that some people are capable of joint digestion, though this procedure had none of the emotional impact of sexual intimacy. Would that show that the rest of us are incapable of "real marriage", since our unions fail to be "comprehensive" in virtue of leaving out the important part of us that is our digestive systems? It'd be awfully peculiar if the reality of my marriage depended upon the absence of this external contingency!)
It seems far more sensible to understand 'comprehensive' in a sense that concerns important human interests, as opposed to mere biological functions. As Jason Kuznicki puts it, "Marriage is a complete, all-encompassing, nurturing relationship. It’s about care for the whole person, so much so that no one else in all the world is quite as important." It's clear why relationships that are comprehensive in this sense are distinctively valuable. And it's equally clear that whether a couple have a relationship of this kind does not depend upon how their genitalia match up.
Test case: Suppose an old heterosexual couple get legally married, and spend the rest of their lives together. They are dedicated to each other's welfare, and share all that they consider important in life. Maybe they even adopt a child to raise together. They kiss, buy each other flowers on occasion, and are sexually intimate in various ways. But, for some reason (perhaps the woman suffers from severe vaginism), they just never have vaginal intercourse. Does anyone really want to say that they aren't really married, or that their relationship somehow fails to fall under the same normative category as a similarly nurturing and (humanly-)comprehensive relationship that happens to involve sharing a "biological function" in addition?
The authors acknowledge that "development and sharing, including the bodily union of the generative act, are possible and inherently valuable for spouses even when they do not conceive children." (p.257) But then why is it not equally true that such development and sharing, including sexual intimacy of a non-'generative' kind, is possible and inherently valuable in just the same way? Why should the biological category of one's actions make any difference to their normative character, if it makes no difference to the interests or well-being of affected persons? The authors' view seems to constitute a kind of bio-functional fetishism.
[Though one may question whether the proffered reasons are what really motivate the authors here. It's telling that when they finally consider the interests of gay persons, on p.283, the authors emphasize the possibility of "non-sexual friendship". I don't mean to denigrate the kinds of "deep friendship" they discuss, but I wonder: why is the option of a comprehensive (incl. sexual) relationship not still on the table?]
[I]n relationships that lack this orientation [towards reproduction], it is hard to see why permanence and exclusivity should be, not only desirable whenever not very costly (as stability is in any good human bond), but inherently normative for anyone in the relevant kind of relationship. (p.259)
But this is not particularly mysterious, on the 'marriage as (humanly-)comprehensive nurturing' view. A transitory relationship is clearly not a comprehensive intertwining of one's lives. And exclusivity seems more of a contingent pragmatic/instrumental consideration (given the possible risk of sexual dalliances distracting or even drawing one away from one's prior relationship), rather than anything essential to good marriages as such. Anyway, one can raise here a general dilemma: Either these features will serve real human interests, in which case their normativity is easily explained without appeal to 'biological functions', or else they don't serve any real human interests, in which case they mustn't be normative after all.
The authors continue (p.261):
Less able to understand the rationale for these marital norms, people would feel less bound to live by them. And less able to understand the value of marriage itself as a certain kind of union, even apart from the value of its emotional satisfactions, people would increasingly fail to see the intrinsic reasons they have for marrying or staying with a spouse absent consistently strong feeling.
This is rather ironic, given that the authors' own arguments rely on a morally incomprehensible bio-functional fetishism. As noted above, if these marital norms are to make sense, they must instead be grounded in real human interests, not metaphysical speculation about biological 'functions' and becoming 'one flesh'.
Won't somebody think of the children?
The authors object that if same-sex marriage were legalized,
the message would be sent that a household of two women or two men is, as a rule, just as appropriate a context for childrearing, so that it does not matter (even as a rule) whether children are reared by both their mother and their father, or by a parent of each sex at all. (p.263)
All the better! I endorse this message, and so should you. I think it's important (as a rule) that children be raised in a stable and loving environment. I do not think that the genitalia of the parents matters.
Throughout the paper, the authors make reference to "sociological evidence [that] children fare best... when reared by their wedded biological parents." (p.257) More specifically, a cited study tells us: "Children in single‐parent families, children born to unmarried mothers, and children in step‐families or cohabiting relationships face higher risks of poor outcomes." (258) But the unsurprising fact that children tend to do worse in broken or unstable families is no evidence at all that gay stable families do worse at raising children than heterosexual stable families. Nowhere in the paper do the authors address, or even acknowledge, this obvious objection to the relevance of the data they cite. One wonders whether they are trying to mislead their readers.
More relevant studies would directly compare straight parents with gay parents (as opposed to comparing stable straight families with dysfunctional straight families). The early evidence that I'm aware of suggests that children raised by lesbian couples face drastically lower risk of abuse, and do outright better on many relevant scales (spanning academics, self-esteem, and behaviour). Perhaps we should ban heterosexual marriage, and restrict the institution to lesbians only?
The article pursued two routes to its anti-gay marriage conclusion: one broadly philosophical, the other more pragmatic. The "philosophical" argument rested on a bizarre and fetishistic appeal to the normative significance of biological functions, losing sight of the kind of "comprehensiveness" that really matters for marital relationships. The pragmatic argument brazenly appealed to irrelevant studies that don't really support their conclusions at all.
And besides the possible impact on children and families, it's worth considering some of the broader implications of the gay marriage debate. One relevant consideration that is thoroughly neglected by the article's authors is that this debate is taking place in a social context of historical - and, often, ongoing - oppression and stigmatization of gay persons. By arguing that same-sex relationships cannot possibly have the same value as heterosexual relationships, the authors are contributing to this stigmatization. They are lending their academic prestige to real world bigotry. The authors worry that their view "will increasingly be regarded as evidence of moral insanity, malice, prejudice, injustice, and hatred." (p.265) But when one considers the flimsiness of their arguments, and the seriousness of these real-world effects, is this really a surprise?
Update: Sherif Girgis responds by arguing that since we care about (i) having biological children, and (ii) rights to bodily integrity, it (somehow) follows that concern for biological functions / neo-Aristotelian teleology isn't fetishistic? I leave the refutation of this silliness as an exercise for the reader.