An argument in favor of homosexual marriage that you are likely to hear again and again on radio talk shows, on national television, and on the Internet, reflects a line of reasoning that you must be prepared to counter. It is embodied in these kinds of questions: Why all the fuss about gay marriage anyway? And why should it matter to you if a gay couple marries and moves into your neighborhood? Why shouldn’t our definition of family be broadened and modernized? After all, what harm could possibly be done by yielding to the demands of those who say traditional...