A publicly reasonable argument for traditional marriage specifies the state interest in terms of sustainable procreation and cultivating in citizens the two moral powers, which are “a capacity for a sense of justice and for a conception of the good.” According to Rawls, a conception of the good is “a conception of what is valuable in human life,” which is comprised “of a more or less determinate scheme of final ends, that is, ends that we want to realize for their own sake, as well as attachments to other persons and loyalties to various groups and associations.” A conception of the good is “fully comprehensive if it covers all recognized values and virtues within one rather precisely articulated system; whereas a conception is only partially comprehensive when it comprises a number of, but by no means all, nonpolitical values and virtues and is rather loosely articulated.” In short, a conception of the good is the coherent narrative of a person’s identity that he develops for himself.
A liberal democratic society needs sufficient children and it needs them to be educated. Therefore, a liberal democratic society needs families headed by two married parents who are the biological mother and father of the children, because such families are (a) intrinsically generative and (b) optimal for childrearing. In other words, sex between men and women makes babies; society needs sufficient babies; babies need moms and dads. Every family arrangement in which children are raised need not and cannot conform to this pattern, but the state has a legitimate interest in encouraging people to form families that do so, which the state can accomplish by enshrining this conception of marriage in the law, as conferring unique social status, and promoting it with material benefits.
1 Br. J. Am. Leg. Studies 411, 438 (2012) (foot notes omitted). As to the opposing argument, O'Brien writes:
... The non- public, moralistic character of arguments in favor of same-sex marriage is often obscured by a rhetorical maneuver, however, which frames the debate as if it were simply about providing equal and fair access to an agreed-upon, uncontroversial social good. In brief, such rhetorical arguments for same-sex marriage proceed as follows. First, “marriage” gets implicitly defined as any affective sexual relationship between two adults. Second, it is argued that since the state promotes “marriage,” it should promote it fairly and with equal respect, not denying access to anyone who is eligible. Third, it is argued that since gays and lesbians can obviously have affective sexual relationships, there is no reason to preclude them from marrying, because to do so would be to discriminate against them as a class. This argument is often quite successful rhetorically, but it relies on a question begging definition of “marriage.”
Mary Lyndon Shanley, for example, begs the question when she says, “Despite their differences, neither side [in the same-sex marriage debate] questions whether marriage is a good thing and whether it should be recognized by the state; their argument is over who should be able to marry.” On the contrary, the debate is precisely about whether marriage, according to its historic meaning, is a good thing or not. Gay rights activists think that marriage, historically understood, is a bad thing because it has the effect of establishing heterosexuality as socially normative, and by implication, they argue that it “inflicts profound psychic damage” on people who embrace a homosexual identity as part of their self image. They propose abolishing marriage and replacing it with a new legal category that solemnizes any affective sexual relationship between any two adults and thus discourages sexual complementarity as a social norm. It is politically useful to call this new category “marriage,” too, because it conceals just how expressively significant the change is, and makes it more likely to convince wary voters to accept the change. But to define “marriage” as a relation equally open to heterosexual and homosexual couples, as Shanley does, is first, simply to beg the question against the natural law defenders of traditional marriage, for whom sexual complementarity is marriage’s sine qua non, and second, to impose an alternative comprehensive doctrine. In other words, the natural law theorists claim that marriage is essentially heterosexual because they claim that only heterosexual sex is valuable. Liberals like Shanley think that any kind of consensual sex is valuable, so they reject the natural law account and want to redefine “marriage.”1 Br. J. Am. Leg. Studies at 456-57 (foot notes omitted). Moreover, he notes, homosexuals want to redefine marriage to exclude any familial unit not based around sex, and the arguments in favor of homosexual marriage ignore the child's interest in his or her parents' marriage as a public good to meet the child's needs. He also observes that "it is absurd to attempt to assess whether some individual or group has a claim on a public benefit, or liability to some public burden, without first determining what the state interest is in offering the benefit or imposing the burden. The nature of the state interest in the family will determine whether and what publicly reasonable arguments are available to justify restricting or expanding access to the legal category 'marriage.'" Id. at 458. Although, the arguments for homosexual marriage pointedly ignore the state's interest.
The ground for such a policy [of only allowing traditional marriage] is, as Rawls argues the ground of any marriage and family policy must be, the permanent and basic social need for orderly reproduction over time. A family headed by two married parents who are the biological mother and father of their children is the optimal arrangement for maintaining a socially stable fertility rate, rearing children, and inculcating in them the two moral powers requisite for politically liberal citizenship.1 Br. J. Am. Leg. Studies at 462.
(H/t The American Conservative)