You don't need the Biblical definition of marriage to know what marriage is

I'm quoting an article that is a couple of years old, back from the debates in California entrenching the traditional view of marriage in law. The proposal put forward was that: Proposition 8, "Only marriage between a man and a woman is valid or recognized in California." In the second paragraph below, the writer talks about how religion did not define marriage, that it's definition is easily discerned from "human nature and it's fulfillment".

Kmiec and the editors of the Times join a long series of activists who insist on framing the same-sex marriage debate as a clash between civil liberties and religion. But that’s not what it is. This debate is about the substantive differences between same-sex marriage and traditional marriage, whatever they are called. The question is whether the substance of the traditional institution should be endorsed both by voluntary associations (including houses of worship) and by the state as the ideal union of adults and the ideal environment for childbearing and childrearing.

Much more than share a television remote, spouses play a crucial public role in any healthy society. Much more than the private union of consenting adults, marriage is vitally important for the well-being of our nation’s children. That’s why Kmiec’s characterization of marriage is unsound. While he is right to note that the state did not create marriage, he is wrong to claim that religion did. Marriage exists as a natural, pre-political, and pre-religious institution based upon human nature and its fulfillment. States and religions rightly recognize and support marriage, but it precedes both. Kmiec, who writes as a Catholic, fails to notice that his argument contradicts the Catholic faith, which teaches that you don’t need the Book of Genesis — or any divine revelation — to know that man and woman are sexually differentiated and that marriage is founded on the bodily union of sexually complementary spouses. Though Catholics believe that Jesus elevated this natural relationship to participate sacramentally in the divine Trinitarian life, this elevation does nothing to eliminate or obscure marriage’s status as a natural human institution. That is why the Catholic church has always regarded the marriages of nonbelievers as true and valid.

Related link: Preserving Marriage in Substance, Not Just Name ~ National Review Online