The ability of married partners to procreate has never been required to make a marriage legal or valid, nor have unwillingness or inability to have children been grounds for divorce.
And marriage, as I have argued, has not been one unchanging institution over time. Features of marriage that once seemed essential and indispensable proved otherwise. The ending of coverture, the elimination of racial barriers to choice of partner, the expansion of grounds for divorce—though fiercely resisted by many when first introduced—have strengthened marriage rather than undermining it. The adaptability of marriage has preserved it.
Nancy Cott approaches the very emotional topic of gay marriage from the best angle possible: the historical/legal one. With magnificent detail, she explains that modern marriage was the result of literally hundreds of years of evolution. Allowing gay marriage will simply be the next step: