A third flaw in the argument is that it presupposes that every divorce constitutes a failure of a couple's marriage. Sometimes this is true; sometimes it is not. I know a couple married for 30 years who made a beautiful home for their three now-married children. The couple divorced last year because they had both concluded that they had drifted too far apart to continue living together in any meaningful way (one aspect of the drift was one partner's increasing devotion to religion and the other's decreasing interest in it).
Who has the hubris to call their marriage a failure? Their children surely don't think their parents' marriage was a failure. It produced three wonderful married adults, and it provided them a beautiful and loving home in which to grow up. One can only wish all marriages so "failed."
It is simplistic to maintain that the one criterion of success or failure in marriage is permanence. There are marriages that provided years of comfort to a couple and a fine home to their children that eventually end; and there are permanent marriages that have provided neither comfort to the couple nor a loving environment for their children. If the end of something renders it a failure, every one of our lives is a failure, since they all come to an end.
Finally, marriage is threatened not by divorce, but by people not marrying in the first place -- as is increasingly the case in the two European societies that have redefined marriage to include couples of the same sex. Our present high divorce rate is not stopping the vast majority of Americans from wanting to marry. Nor should it. Nothing provides the antidote to narcissism, or the environment for the healthy raising of children, or the way for people to take care of one another, as does the marriage of a man and a woman. And while most divorces are terribly sad, divorce itself no more undermines the institution of marriage than car crashes undermine the institution of driving. In fact, the vast majority of people who do divorce deeply wish to marry again; painful divorce has not undermined marriage even among those who have divorced.
There may be honest reasons to support the redefinition of marriage to include same-sex couples. The argument that heterosexuals divorce a lot is not one of them. It is, in fact, demagoguery.