Ross Douthat, one of the NY Times conservative columnists, has a pretty grounded, yet fantastic opening to his weekly column about the Gay Marriage Ban reversal in CA.
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 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.
In other words, the definition of marriage has been changing for some time, and will continue to do so.