If you listen to contemporary conservative Christians who are opposed to gay marriage, you might get the idea that Christianity teaches that the only legitimate purpose for sex is procreation and that marriage exists to sanction sex. Whether you listen to quasi-Christian populists like Ralph Reed, James Dobson, or read statements such as the Manhattan Declaration, the notion that marriage is structured around procreation is taken as axiomatic. In the Christian tradition, however, the meaning of marriage is more than an open question, and certainly not as dogmatic as they make it seem.
In the Manhattan Declaration, a pronouncement signed by Orthodox, Roman Catholic and Protestant clergy and laity alike, they allude to "Christian tradition" as dogmatically expressing the notion that the purpose of marriage is to sanction the sexual act for the purpose of procreation. Having children, therefore, is the fulfillment and meaning of marriage.
A natural response is to ask about the legitimacy of marriages for infertile couples, given the assumption that procreation both defines and fulfills marriage, and the Declaration has an answer. The marital commitment between a man and a woman, they claim, is consummated by a single purpose, which is expressed by "fulfilling together the behavioral conditions of procreation" (emphasis mine). In other words, marriage is to be structured around having sex in order to procreate, whether you can actually procreate or not, which legitimizes sex within the marriage.
So, according to the signers of the Declaration who come from these three very disparate traditions, Christianity teaches that even if you know that as a couple you cannot have children, as long as your marriage is structured around the desire to have children through sexual intercourse (i.e., as long as you have the right structure, the results are unimportant), your marriage is sanctioned according to its purpose even if you cannot possibly fulfill its purpose. Quickly, such reasoning dissolves into nonsense.
One might argue in response, tongue-in-cheek, that rather than defending the "Christian institution of marriage" from the legalization of same-sex unions, it opens the door. Any same-sex couple who agrees to behave as an infertile couple, structuring their marriage around sex, and sex around an impossible procreative act, can therefore make their marriage legitimate in the eyes of the Church as well. Narrowly defining behavioral conditions in terms of genitalia quickly can be reduced to absurdity whether we are talking about an infertile couple, or two members of the same sex. At the very least, the contorted logic encourages cognitive dissonance for infertile couples if not outright irrationality.
Fortunately, it isn't necessary to further twist logic in convoluted arguments because there is no conciliar dogmatic pronouncement on the subject, and it is not, contrary to what is inferred in the Manhattan Declaration, a dogma, such as is, say, the dogma of the Trinity, or of the two natures of Christ. Secondly, many recognized theologians and laity throughout history and in many different Christian communions today have a very different perspective.
I'm most familiar with the Orthodox Church, of which I am a member. I agree with the noted Orthodox theologian, Paul Evdokimov, who writes, "Both the preservation of the species and selfish sexual pleasure reduce the partner to a mere tool and destroy his dignity. Love alone bestows a spiritual meaning upon marriage, and justifies it by elevating it to perceive the countenance of the beloved in God, to the level of the one and only icon..."
According to my understanding of the Orthodox view, the purpose of marriage is not motivated by survival of the species, the urge or mandate to procreate, nor merely to enjoy the pleasures of sex. While both procreation and sexual enjoyment are of course present in most marriages, they do not define what it means to become "one flesh." Rather, this unity of purpose between two persons, which has historically been between males and females, retains its purpose, and therefore its significance, in the mutual progression towards personal transformation and salvation. This is the mystery of marriage within the context of the Orthodox Church. Other perspectives, such as marriage as a convenient contractual arrangement, are socially constructed and derived. The Church takes the vehicle of the social construct, such as marriage (in this instance), retains its value and elevates it to a higher purpose. To confuse the sacrament with the vehicle is like confusing the Eucharistic gift with bread sold on the shelf at the local supermarket. Evdokimov continues:
"Man and woman move toward one another by 'mutually getting to know each other,' by revealing themselves to each other for a shared ascent; nothing comes to ennoble or legitimize, still less to 'pardon' this meaning that royally imposes itself before, or even independent of, procreation....It is from this overflowing fullness that the child can come as fruit, but it is not procreation that determines and establishes the value of marriage. St. John Chrysostom says: 'When there is no child, will they not be two? Most certainly, for their coming together has this effect, it diffuses and commingles the bodies of both. And as one who has cast ointment into oil, who has made the whole one, so in truth it is also here.' '...Marriage is the intimate union of two lives, ' 'the sacrament of love.'"
Love is therefore the unitive factor in marriage, and not the structure of sexual compatibility nor the capacity to produce offspring. The 19th Century Russian writer (and friend of Dostoevsky), Vladimir Solovyov, reiterates the point. In an essay titled, "Beauty, Sexuality and Love" he writes, "The meaning of sexual love is generally supposed to consist in subserving the propagation of the species. I consider this view to be mistaken -- not on the ground of any ideal considerations as such, but first and foremost on ground of natural history."
Solovyov's appeal to nature is at least as compelling as that of those who would argue that same sex unions are immoral due to a contrariness to nature. He first makes the point that sex is not necessary for reproduction, which may be witnessed in processes such as cellular division, sporing and budding, both in vegetable life and in lower animal life forms. Given this, Solovyov posits that "...among animals that reproduce themselves solely in the sexual way (the vertebrates), the higher we go in the organic scale, the more limited is the power of reproduction and the greater the force of sexual attraction" and that therefore ""sexual love and reproduction of the species are in inverse proportion to each other: the stronger the one, the weaker the other....until at last, in man, there may be intense sexual love without any reproduction whatever."
The meaning of sexual reproduction among higher organisms therefore through mutual desire and love brings into being a greater range of meaning and significance than the singular, instinctual purpose of propagation, but rather the same mystery that unites Christ to his bride, the Church, which is love.