On the other hand, we have people sympathetic to same-sex marriage who offer definitions along the lines of "a marriage is two people who are in love and live together". But this too fails to satisfy for similar reasons. We certainly know of people who love and live together who are not married, and sadly, we probably know people who are married who are not in love or do not live together. Moreover, this definition makes marriage sound like it is only about self-fulfillment, about "finding love", which is a selfish purpose. And I hope we all agree that marriage is not about being selfish, in fact it's the opposite: marriage is about a specific kind of altruism and responsibility.
It is indeed strange that rational discourse could become so fuzzy about a concept that is so clear in everyday society. In everyday society, the concept of marriage is quite unambiguous and well-agreed. There is no dispute about whether a particular couple is married or not, or if there is any dispute, we all know how to settle it. When a couple is married, we can all point to an exact date on which they became married. After that date they are married, and before that date they were not. Nothing fuzzy about it. This is because marriage is distinctly demarcated by a particular ceremony called a "wedding", which consists in two people exchanging vows of lifelong loving commitment. Or in other words, marriage commences when we hear two people say "I do". We know with certainty that a couple is married because we witnessed their exchange of vows, or we can find someone who witnessed it, or we can find some record of it. Thus, we can start to identify the necessary and sufficient conditions for recognizing a marriage:
- mutual exchange (it doesn't count if only one person says "I do")
- public (someone has to witness the vows)
- lifelong ("till death do us part")
- loving commitment ("to love, honor, and cherish...")
Better than this, we have a pretty clear idea what "lifelong loving commitment" entails, because it is eloquently synopsized in the language of the vows: "for better or worse, for richer or poorer, in sickness and in health". In other words, I stick with my spouse whether he wins the lottery or loses his job. If he suffers a debilitating stroke, I stick around to take care of him. For tomorrow, next week, next year, through our youth and our old age. (It's interesting to note that childbearing, that alleged sine qua non of marriage, is often not even mentioned in the liturgy of marriage, and mentioned obliquely if it's mentioned at all.)
One might ask whether a state license is a necessary condition of marriage. Generally, this question isn't faced as the vast majority of marriages include licenses. However, it is important to make the inquiry. I would respond with several observations. We can conceive of a couple getting married by the captain of a ship in international waters, who may not have a license from any state, and yet still we would consider them married. We can conceive of an elderly couple wishing to have a quiet wedding ceremony performed by their pastor, and explicitly not wanting the ramifications of a legal marriage to mess up their trusts or social security benefits. (Our pastor tells us this happens with some regularity.) Of them, we might say something like "they are married, but not legally". We know examples of couples getting married against the wishes of their parents and their state (Romeo and Juliet, for instance), and yet they are still married. Even the Supreme Court has acknowledged that marriage is prior to our Constitution and laws (see Justice Douglas in Griswold v. Connecticut, for instance). Thus, I would conclude that a state license is not a necessary condition for marriage.
Given this analysis of marriage, it should now be clear that the permutations of genitalia of the participants have no essential part in constituting marriage. We can easily conceive (and probably know) of two persons of the same gender who have publicly exchanged mutual vows of lifelong loving commitment, and are living out those vows. Such instances do no damage to the concept of marriage, and in fact reinforce it by further examples in practice. The answer to the question of whether two persons of the same gender can marry should be clear: We can. We have. We do.