In the debate about gay marriage, it is often argued that the purpose of marriage is to provide the best context for raising children. Thus, the argument goes, marriage is intrinsically heterosexual, because gay couples (in and of themselves) can not procreate. There are a number of things that can be said in response to this. The most common response is to note that defining marriage solely in terms of procreation demeans the marriages of those who are infertile and of those who choose not to have children. Most Americans would find it cruel (needless to say incorrect) to assert that the marriage of an elderly couple or a childless couple is pointless. Instead, we should acknowledge that childrearing is a purpose -- but not the only purpose -- of marriage. (And I should like to write more about other purposes another time.) But the most insidious fallacy of the "marriage = procreation" argument is the conceptual confusion between childbearing and childrearing. (This confusion runs so deep that even normally rigorous thinkers such as the Anal Philosopher mix up the concepts.)
Procreation is the act that leads to bringing a new child into the world. Childrearing is the provision of love, support, guidance, and resources over nearly two decades. While childrearing naturally and optimally follows procreation, the two acts are distinct and separable. Our society is painfully aware that the ability to produce children does not necessarily coincide with the ability to raise them. Sadly, many children are born to parents who are not prepared or able to take care of them. Happily, many couples are willing to adopt children which are not their own, a act of great altruism generally viewed as admirable. In fact, adoptive parents are likely better equipped and on average more successful at childrearing than natural parents, since there are no unintentional adoptive parents, and (unlike natural parents) adoptive parents must interview for the job.
Now that we have distinguished these concepts, we must ask whether marriage is for procreation or for childrearing. And I think we would have to answer that procreation (a one-night act) is secondary to childrearing (a two-decade act). It is the long haul of childrearing that the institution of marriage is well-designed to support. Naturally we like to see the two go together, but given a choice between a family of adoptive parents versus two people who accidentally produced a baby, it should be clear in which context the essence of marriage belongs.
Finally we return to the question of whether marriage, insofar as it is for childrearing, is intrinsically heterosexual. Granted, procreation is intrinsically heterosexual (although some homosexual couples do procreate by means of more complex arrangements involving third parties). But childrearing -- which we have said is the provision of love, support, guidance, and resources over a long haul -- does not logically entail heterosexuality. Many homosexual couples have proven to be competent parents. (Here is one example where a Florida family court judge praised two men for being model parents.) While some will claim that proper child development somehow requires both male and female models, it is a narrow view of the world that insists that only men have stereotypically "male" attributes and only women have "female" attributes. Moreover, unless the family lives in a cave, the children will meet other people of both genders (extended family, teachers, friends) who will have some part to play in shaping their understanding of the world. What I think most people can agree is truly essential is to provide a stable, secure, and loving home. And it is the marriage of two loving committed spouses -- straight or gay -- that is best suited to provide that environment.