Most people would choose to kill the termites that infest the wood, and replace the boards as best they could. But the situation described is analogous to the state of marriage in the First World, and activists have chosen (metaphorically speaking) to stockpile matches and gasoline.
Activists seeking to expand marriage to include same-sex unions snidely claim that “heterosexual Christians have already killed marriage” – and they are absolutely correct (if slightly hyperbolic) in claiming that many Christians already treat marriage as a joke. Rampant divorce, adultery, extramarital sex – these are symptomatic of a radically deformed view of marriage, ever-present even among Christians.
But the way to combat the evils afflicting marriage is to concentrate on reducing those evils by encouraging fidelity. Instead, activists have taken the opposite tack, supporting the full-scale destruction of “restrictions” on marriage and celebrating those very cultural influences which mock and undercut marriage. They use the resulting damage done to the institution of marriage by their efforts as an excuse to fundamentally change marriage by expanding it.
Changing marriage necessitates a radically different view of marriage than the traditional view; the new view of marriage fundamentally alters its essence. Marriage becomes a contract between any two partners in a “special” relationship based in a fuzzy notion of “love,” not the foundation of the family and the social order. This leads to radical changes in the nature of marriage; the same arguments for same-sex marriage could be marshaled for allowing incestuous marriage.
Advocates of a more inclusive definition of “marriage” don’t want to save marriage. They want marriage reduced to a shell of itself; where only the word marriage and its framework remains, while its essence is changed into something unrecognizable.
Of course, problems don't go away when you embrace them as solutions.