Philosophers are too clever by half. When it's asserted that marriage is a childrearing institution, i.e., that the purpose of marriage is to provide for children, the retort is that not all married couples have children--or even intend to upon marrying. This is supposed to refute the assertion.
It does nothing of the sort. What it does is reflect a misunderstanding of the nature of law. Law is necessarily crude. [...] A rule that restricts marriage to heterosexuals is much less costly to implement than one that restricts it to those who have or intend to have children. This is not to imply that all heterosexuals have children (although most do) or that no homosexuals have children (most do not). It's to make a distinction that correlates with what matters but is less costly to implement than alternatives.
To try to clarify it, one could formalize his argument as follows:
1) What matters is that "all and only those with children should be allowed to marry".
2) A good law "make[s] a distinction that correlates with what matters but is less costly to implement than alternatives."
3) Most heterosexuals [seeking marriage] will have children, whereas most gay couples [seeking marriage] will not.
4) "A rule that restricts marriage to heterosexuals is much less costly to implement than one that restricts it to those who have or intend to have children."
C) Therefore, it is a good law to restrict marriage to heterosexual couples only.
I've tried to be charitable in my interpretation of KBJ's argument here. However, if anyone thinks I've misinterpreted, I'm open to suggestions for improvements. But for now I will assume this is indeed his argument. So, how does it look? In brief: premise one is false, premise three dubious, and premise four (though true as it stands) is too weak for the conclusion to logically follow, and the strengthened version will no longer be true. Let's have a closer look.
The argument's logic requires that banning gay marriage be "less costly to implement than alternatives." But premise four fails to establish this, for it only mentions one alternative: restricting marriage to "those who have or intend to have children". Perhaps there is another, better, alternative. It may have been uncharitable of me to construct an invalid argument. So I'll fix up premise 4 to say what KBJ probably should have argued:
4a) A law that bans same-sex marriage is "less costly to implement than alternatives".
(Presumably costs are assessed in proportion to the measure's correlation with childrearing, so that a small increase in cost would be acceptable in order to achieve a much stronger correlation with the goal.)
Now, I see very little reason to think that this new premise is true. For example, it seems plausible that a stronger correlation to childrearing would be found using age rather than sexual orientation. Anyone over the age of sixty (say) seeking to enter a new marriage, is very unlikely to be doing so for the purposes of childrearing. Yet age is a matter of formal record, so can be easily assessed by the law. It would be no more 'costly' (in this technical sense) to ban geriatric marriage than it is to ban gay marriage. And that's just one example; the possibilities are endless. An even more obvious alternative is to simply allow any couple who wish it to get married (i.e. the liberal position). Using voluntarism as our method of 'distinction' has the lowest cost of all, and probably still correlates reasonably well with childrearing anyway.
As for premise three, I've already made an alteration for the sake of validity. It seemed that KBJ merely argued that most homosexuals do not have children. Which may well be true. But here we're only concerned with those couples that are seeking a marriage. And when considering only marriage-seeking couples, I'm simply not convinced that sexual orientation would be highly predictive of childrearing intentions. It seems to me that for those who are looking to 'settle down', raising children might often be a part of that ideal, even for gay couples. But I admit I'm just guessing here, we would really need some empirical data to settle the matter. (Incidentally, given how I've reworked the fourth premise to include the degree of correlation, I suppose premise 3 would now be subsumed under 4a.)
Premise 1 appears to be the core idea behind this whole argument. And I really don't find it at all plausible. Why should marriage be ideally restricted to only those with children? How is that ideal? If a childless couple wish to celebrate their love and commitment by getting married, how is that a bad thing? I don't understand. And I doubt anybody else does either.
Here's the crucial point:
KBJ has failed to understand the import of the "childless straight couple" objection. According to him, the problem is that the law is a blunt instrument. That is, grand though it would be to exclude childless straight couples from marriage, there is (alas?) no easy way to achieve this in the real world.
But here's the real problem: Does anybody actually believe that claim I put in bold? I mean, seriously, when you see a childless (but straight) married couple, do you say to yourself, "Damn, another undeserving couple slipped through the legal loophole"? Do you truly believe that they really, ideally, ought not be married? Imagine if infertile couples could be easily identified and prevented from marrying. Would you really consider that a good thing? Surely not! Yet that is precisely what is implied by KBJ's argument here.
Childless straight couples are problematic for this anti-gay argument not because we tolerate such marriages, but because we cherish them. Banning such marriages is not some 'impractical ideal', it is not ideal at all! Given this, how could one consistently appeal to this ideal to justify banning gay marriage? I submit that you cannot, and the attempt to do so is simply a feeble rationalization for bigotry. As always, I invite reasoned arguments to the contrary.
KBJ suggests that philosophers have neglected to take into account the crudeness of law (premise 2) when they should, and that this must be why they disagree with him. He couldn't be more wrong. We can all accept that the law is a blunt instrument. What's wrong with his argument is everything else.
P.S. For more on the topic, see my post: Gay Marriage Arguments.