O.K., I think we are actually making progress. Progress, I mean, towards the goal of achieving disagreement. I'm no Socrates. I have no illusions that if you accept certain opening premises (like, say, the broad and deep historical connection between marriage and procreation), down the road you are going to have to say, "Gee, Maggie, only a fool could disagree."

One of the signs of progress towards disagreement is the shift among commentators to another key question: well, o.k. Maggie, so just how exactly will SSM threaten procreation? It is an important question. And I'm going to answer it. But not yet.

I want first to continue on the foundational question: why I think procreation is the main public purpose of marriage, in the sense of being the driving force behind its unique legal status (Remember, individuals or religions may have other reasons; I'm talking about why government in our system is involved in marriage and not other intimate adult relations).

Let me offer 2 additional reasons why I believe in the primacy of procreation (not, remember, as the definition or the essence of marriage—marriage is a sexual, financial, emotional union of husband and wife) but as the main reason why marriage exists as a legal status and as a universal human idea.

1. Internal coherence. The first reason is internal evidence from the structure of marriage in our received legal tradition. Procreation provides the "coherence" for our received definition, the thing that explains its core features, in way that other reasons do not. The California Baker court in 1859 laid out the the prime reason as procreation and a second reason as the happiness of the couple. Fair enough. But how does the happiness of the couple explain why two and only two people can get married? Or why two people, in order to be happy, must have sex?

By contrast, procreation (or to be more accurate, managing the procreative consequences of sexual attraction between men and women) provides an answer to all these questions: Why two? Because two and only two people make the baby. Any other arrangement involves not a full union of parents but a subdivided one. Why a sexual union and not some other loving kind? Because sex makes babies.

One of the problem SSM advocates have is coming up with an alternate reason for marriage that explains why, out of all the intimate relations adults forms, only certain kinds of interpersonal unions are eligible to be marriages. Marriage is about love they say. Legally, love defines marriage even less tightly than procreation. You can love your mistress and hate your wife, and that doesn't mean your mistress is your wife. You can love many people but only have one wife. Why? And why do you have to have sex with people you love? (Note: I'm not arguing that SSM will lead to polygamy. Personally, I think it will lead to de-institutionalization of marriage altogether, not to polygamy.Instead I'm pointing out that under their theory of why marriage exists, advocates of SSM cannot explain any of marriage's ordinary legal features).

2. Universality. Many features of our specific marriage system are not universal. Monogamy for example, is the exception in human history (although most large complex societies have this marriage system). But all of these systems (with very few and very limited exceptions) define marriage as male and female. Why? You don't find that many human universals.

BTW this is true even in the many small tribal societies that institutionalize and approve same-sex (male) relations in many contects. That is these societies also seem to reserve marriage for those relations that are as Kingsley Davis said socially approved for sexual intercourse and baby-making.

The argument I am making is this: every society needs to come up with some solution to the fact that the default position for male-female sexual attraction (that is unregulated by law or society) is many children in fatherless homes. The second human reality societies must face is that procreation is not optional, it is necessary. Individuals don't have to do it but societies do. The word for the social institution that addresses these problems, in this and every known human society is marriage.

Sex makes babies, Society needs babies, babies need mothers and fathers.

I think there is powerful evidence that these "facts on the ground" really do explain marriage in some sense better than any alternative explanation on the table.

Next post: how these facts on the ground affects the legal arguments for SSM.