Thus gay marriage opponents' persistent disadvantage. They can argue from tradition, custom and Christianity -- as Obama himself does, albeit with dubious sincerity, to explain why he backs civil unions but not full-fledged marriage. They can note the perils of formally severing the link between marriage and childbearing in a society where far too many children are born outside of wedlock as it is. But supporters of gay marriage are the only ones making an argument from personal liberty -- the freedom to marry, the right to marry -- and that has made all the difference.