Matthew Franck argues that the rush to redefine marriage is fueled in part by the weak arguments underpinning the movement:

"...In the thick of the struggle over the law and politics of marriage, we can easily forget how novel is the idea of two men or two women marrying each other.

This fact came home to me when I participated in a forum on the subject at Princeton University last spring. Present in the room were two lions of the liberal academy, each past the “threescore years and ten” of which the Psalmist speaks, each a distinguished scholar with many publications to his credit, each known for his devotion to liberal causes. Both gentlemen expressed the opinion that the cause of same-sex marriage was obviously just, that opponents of the cause were obviously reactionary and benighted, and that this was plainly the new civil rights struggle of our time.''

Yet it struck me that if denying same-sex couples the “right to marry” was such an obvious and gross injustice as to merit such energetic claims today, why had it never occurred to either of these august scholars decades ago, at the beginning or the middle of their careers? In the books of proud advocacy each had published, say, twenty or thirty years ago, there was not the slightest hint that American public life was disfigured by this particular injustice.

Redefining marriage to include same-sex relationships simply didn’t occur to them, because it didn’t occur to anyone. Yet that day they espoused that view with the fervor of men who had always thought so, and for whom it was unthinkable to believe otherwise. If they reflected on this change in their own thinking, would they conclude that their reasoning powers had been deficient years ago, or their moral sympathies inadequate?" -- Public Discourse