Keith Burgess-Jackson writes,
The purpose and point of marriage, as everyone knows, and as the law has long recognized, is to bind a man and a woman to one another for the sake of any children they produce. Please, please, please don't say that infertile heterosexual couples are allowed to marry, as though that refutes my claim. Immature 21-year-olds are allowed to drink alcohol, but that doesn't mean the purpose and point of the drinking age isn't to prevent immature individuals from drinking alcohol.
That's right. As I say near the end of a long entry, The Infertility Argument for Same-Sex Marriage:
The law cannot cater to individual cases or even to unusual classes of cases. Consider laws regulating driving age. If the legal driving age is 16, this is unfair to all the 13-16 year olds who are competent drivers. (E.g., farm boys and girls who learned to operate heavy machinery safely before the age of 16.) If the law were to cater to these cases, the law would become excessively complex and its application and enforcement much more difficult. Practical legislation must issue in demarcations that are clear and easily recognized, and therefore 'unfair' to some.
But a better analogy is voting. One is allowed to vote if one satisfies quite minimal requirements of age, residency, etc. Thus the voting law countenances a situation in which the well-informed and thoughtful votes of mature, successful, and productive members of society are given the same weight as the votes of people who for various reasons have no business in a voting booth. We don't, for example, prevent the senile elderly from voting even though they are living in the past out of touch with the issues of the day and incapable of thinking coherently about them. We don't exclude them or other groups for a very good reason: it would complicate the voting law enormously and in highly contentious ways. (Picture armies of gray panthers with plenty of time on their hands roaming the corridors of Congress armed with pitchforks.) Now there is a certain unfairness in this permissiveness: it is unfair to thoughtful and competent voters that their votes be cancelled out by the votes of the thoughtless and incompetent. But we of the thoughtful and competent tribe must simply 'eat' (i.e., accept) the unfairness as an unavoidable byproduct of workable voting laws.
In the same way, whatever residual unfairness to homosexuals there is in allowing infertile oldsters to marry (after my foregoing arguments have been duly digested) is an unfairness that simply must be accepted if there are to be workable marriage laws.