One has to admire the proponents of I-957 in Seattle, which ties the legitimacy of marriage to actual procreation, if for no other reason than their willingness to use conceptual entailment as a consideration in the formation of policy, in spite of the fact that their attempt is a rather lame one intended not to actually bring logical consistency to legal policy but rather something quite different: to embarrass their opponents into abandoning legal restraints requiring marriage to be heterosexual-only. At the same time, one cannot help but smile at the conniptions the opponents of the proposal are having, since they could make the whole thing go away fairly easily if they wanted to, while at the same time putting an ironic twist into the ending by flummoxing the proponents of the initiative.
Whenever I teach Aristotelian metaphysics and introduce the notion of essential natures, some student will invariably bring up what he thinks is an obvious and final nail in the coffin of such views: particulars often fall far short of their putative essential natures in terms of their actualized capacities. For example, it is part of the essential nature of human beings to be rational and bipedal, but the sort of student I'm talking about appears to take great delight in point out that some people are actually born with handicaps, either mental or physical, that make it clear, at least to this putative student, that there can be no such things as "essential natures". Stupid old Aristotle, why didn't he think of that? It just goes to show you how easily 2400 years of the Western Intellectual Tradition can be toppled by a single act of 18 year-old intellectual illumination. But I digress. The difficulty with this objection is that it ignores (or probably just forgets, if it ever knew) the distinction between a set of capacities that define an essential nature and the physical working out of those capacities in a material being. For example, we may think of a recipe as something like an essential nature of something like a cake. If I want to see what, say, a German chocolate cake essentially is, I can turn to the recipe in the cookbook. There are the ingredients, the instructions, and maybe even a beautiful full color picture, all glistening with frosting and nuts and maybe with a slice tantalizingly taken out and placed on a plate...OK, Carson, get a grip on yourself. Anyway, we all know from sad experience that when we try to produce what is in the book we often fall far short (especially when the book is a very expensive one). It does not follow from this failure to produce what the recipe says that the recipe does not exist. The essential nature of the kind homo sapiens is rational and bipedal, but when that form is enmattered, the vicarious nature of matter can interfere with the way in which the essential nature gets manifested and you can find all sorts of variation in the material particular. This is, in fact, an important element in Aristotelian metaphysics: matter is the principle of individuation. If matter did not produce variety then all human beings would be perfectly identical. The unfortunate side of this is that some variation is not optimal when it comes to functionality.
Anyway, suppose we define "marriage" as a state in which procreation is the final cause. This means that having procreation as a purpose is a part of the essential nature of the state of marriage. Does it follow from this that any state in which there is no actual procreation fails to be a marriage? The answer is no. Only when there is no capacity for procreation would there fail to be a marriage. There need not be any actual procreation at all for the essential nature to be realized in a given enmattering. Those who propose limited marriage to heterosexual couples are proposing, in their own lame way, that homosexual marriages lack a capacity that heterosexual marriages do not. I say this is lame because, of course, one could easily quibble over how to define this capacity. Should we say, for example, that infertile people ought not to be allowed to marry? Well, if we are to be Aristotelians about this, we don't need to say that, because we could always point out that male and female, taken as kinds, by definition have the capacity to be fertile as an element of their own essential natures, whether or not any particular instantiation of the kind does. But this does not remove the arbitrariness of granting legal recognition only to procreation-ready marriages.
This brings me back to the point with which I ended my first paragraph. This whole argument depends on the state recognizing certain forms of friendship. Now, conservatives find themselves in something of a predicament here, because they tend to favor the status quo and while they approve of heterosexual marriage they do not approve of homosexual marriage, and the reasons for both the approval and the disapproval tend to be rather arbitrary and ill-defined. For some (for example, Christians), the reasons may be religious, but it is no less arbitrary for the state to favor one form of friendship over another for religious reasons than to do so for purely aesthetic reasons. Non-religious conservatives have often suggested that the state has an interest in procreation, since it tends to work in favor of the continued existence of the state, but this reason will fail to defend heterosexual only marriage, since as long as some marriages are heterosexual then this goal will be available, and it is remarkably implausible to suggest that by recognizing homosexual marriage we diminish the likelihood that heterosexuals will continue to reproduce. The answer, it seems to me, lies in the state not recognizing any kind of marriage at all. I really don't see that it is any of the state's business to recognize my, or anyone else's, marriage. Marriage, as such, is Sacramental, and need be recognized only by the Church. What should be at issue is not "marriage", but some kind of contract between friends to live together under certain conditions of joint property. That sort of thing is already widely available in most states, and in fact it is often handled without state intervention. Ohio does not mandate recognition by employers of domestic-partner relationships, but Ohio University recognizes them anyway. In fact, I was on the Faculty Senate when they voted on it, and the faculty were practically falling all over themselves to make their support of the recognition clear, going so far as to vote to suspend the rules of procedure so as to bring the proposal for recognition to a vote three months earlier than would ordinarily have been possible. There were two people who tried to present arguments against the proposal, and they were ridiculed and literally shouted down. So it is implausible to suggest that such recognition will never come unless the state intervenes.
This sort of solution will flummox the proponents of I-957, I think, because they don't want the state to bow out of "marriage" recognition entirely, they want to get domestic partnerships recognized as marriages. I may be wrong about this, but I myself don't see anything about domestic partnerships that Christians ought to oppose, other than the whole sex-outside-of-Sacramental-marriage thing. But that is a problem not only with homosexual domestic partnerships but with heterosexuals who live together outside of matrimony. It's not a "gay" problem in the first place, so it's not a problem with gay domestic partnerships, and the state has no business regulating it.