Not so liberals. There is no biblical or Western basis for redefining marriage. There is only the individual liberal's high self-esteem: He is so compassionate, so enlightened and so decent that his opinion must surely prevail. The conservative, on the other hand, asks what the founders of his society really meant, what the Bible says, and, yes, what does the citizenry want.
Liberals don't ask such questions. They find the question of what the founders' views were to be irrelevant or even contemptible (the founders, after all, are frequently viewed as economics-driven, white, male slave holders); they rarely care what the Bible says, since they regard it as neither divine nor morally compelling; and they have contempt for the citizenry, as it contains a large number of fools (i.e., non-liberals) to be feared for their lack of enlightenment and education.
What liberals ask is how they themselves feel, not what a text or a founder or a religion teaches. This is one reason liberalism is so attractive. One need not know, let alone wrestle with, prior texts or values. One need only consult one's feelings to know what is right (hence the liberal preoccupation with the word compassion). Indeed, to the extent prior texts are studied, they are done so not to learn from but to deconstruct, i.e., to delegitimize.
Therefore, with no prior religious or national value system to restrain them, liberals are free to invent morality in the image of their hearts.
That is how three people can effortlessly redefine marriage. They felt like it.
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