That’s the title of an insightful Public Discourse essay by political scientist Carson Holloway. I’ll quote here just the last paragraph, which applies equally to other efforts to redefine marriage:
Contemporary liberals tend to be environmentalists. In that capacity, they often warn us that even apparently small changes in the natural environment—such as the extinction of a single species—can have far-reaching and damaging consequences for the whole system. They commonly refuse to see, however, that the same concerns apply when we are making changes to our society. In changing the definition of marriage, they think they are accomplishing nothing but an extension of principles they think are good—equality, or sexual liberation. They refuse to acknowledge that they may also be changing the character of our whole civilization.