One of the smartest men I know of, Robert P. George from Princeton University, with Sherif Girgis and Ryan T. Anderson have written an argument for the traditional definition of marriage. You can download the PDF and digest it. The summary reads:
In the article, we argue that as a moral reality, marriage is the union of a man and a woman who make a permanent and exclusive commitment to each other of the type that is naturally fulfilled by bearing and rearing children together, and renewed by acts that constitute the behavioral part of the process of reproduction. We further argue that there are decisive principled as well as prudential reasons for the state to enshrine this understanding of marriage in its positive law, and to resist the call to recognize as marriages the sexual unions of same-sex partners.
Besides making this positive argument for our position and raising several objections to the view that same-sex unions should be recognized, we address what we consider the strongest philosophical objections to our view of the nature of marriage, as well as more pragmatic concerns about the point or consequences of implementing it as a policy.
They make an argument that requires no appeal to religious authority.
It has sometimes been suggested that the conjugal understanding of marriage is based only on religious beliefs. This is false. Although the world’s major religious traditions have historically understood marriage as a union of man and woman that is by nature apt for procreation and childrearing, this fact only suggests that no one religion created marriage. Instead, the demands of our common human nature have shaped (however imperfectly) all our religious traditions to recognize this natural institution. As such, marriage is the type of social practice whose basic contours can be discerned by our common human reason, whatever our religious background. As we argue in this essay for legally enshrining only the conjugal view of marriage, our arguments will require no appeal to religious authority.
(HT: Frank Beckwith)