Sam Schulman wrote a very interesting article for Commentary on secular reasons for keeping marriage between men and women only:
JAMES Q. WILSON, Maggie Gallagher, Stanley Kurtz, and others?including William J. Bennett in The Broken Hearth (2001)?are right to point to the deleterious private and public consequences of instituting gay marriage. Why, then, do their arguments fail to satisfy completely? Partly, no doubt, it is because the damage they describe is largely prospective and to that degree hypothetical; partly, as I remarked early on, the defensive tone that invariably enters into these polemics may rob them of the force they would otherwise have. I hardly mean to deprecate that tone: anyone with homosexual friends or relatives, especially those participating in longstanding romantic relationships, must feel abashed to find himself saying, in effect, ?You gentlemen, you ladies, are at one and the same time a fine example of fidelity and mutual attachment?and the thin edge of the wedge.? Nevertheless, in demanding the right to marry, that is exactly what they are….
The truth is banal, circular, but finally unavoidable: by definition, the essence of marriage is to sanction and solemnize that connection of opposites which alone creates new life. (Whether or not a given married couple does in fact create new life is immaterial.) Men and women can marry only because they belong to different, opposite, sexes. In marriage, they surrender those separate and different sexual allegiances, coming together to form a new entity. Their union is not a formalizing of romantic love but represents a certain idea?a construction, an abstract thought?about how best to formalize the human condition. This thought, embodied in a promise or a contract, is what holds marriage together, and the creation of this idea of marriage marks a key moment in the history of human development, a triumph over the alternative idea, which is concubinage.
I think it’s very well thought out, but ultimately unconvincing.