Here are some commonplace arguments against gay marriage: Marriage is an ancient institution that has always been defined as the union of one man and one woman, and we meddle with that definition at our peril. Lifelong heterosexual monogamy is natural; gay relationships are not. The nuclear family is the universal, time-tested path to forming families and raising children.

These have been losing arguments for decades now, as the cause of gay marriage has moved from an eccentric- seeming notion to an idea that roughly half the country supports. And they were losing arguments again last week, when California’s Judge Vaughn Walker ruled that laws defining marriage as a heterosexual union are unconstitutional, irrational and unjust.

These arguments have lost because they’re wrong. What we think of as “traditional marriage” is not universal. The default family arrangement in many cultures, modern as well as ancient, has been polygamy, not monogamy. The default mode of child-rearing is often communal, rather than two parents nurturing their biological children.

Nor is lifelong heterosexual monogamy obviously natural in the way that most Americans understand the term. If “natural” is defined to mean “congruent with our biological instincts,” it’s arguably one of the more unnatural arrangements imaginable. In crudely Darwinian terms, it cuts against both the male impulse toward promiscuity and the female interest in mating with the highest-status male available. Hence the historic prevalence of polygamy. And hence many societies’ tolerance for more flexible alternatives, from concubinage and prostitution to temporary arrangements like the “traveler’s marriages” sanctioned in some parts of the Islamic world.