Notwithstanding a compelling and growing body of evidence that homosexual/bisexual proclivities extend well into the animal kingdom and serve an important social purpose, we might take this opportunity to reconsider the scope and purpose of marriage in modern Western society: if, in fact, as the Galloways' premises indicate, marriage is a contract predicated upon "replenishing the population," then theirs ought to be annulled.
Of course, this is both a cold-hearted and disingenuous notion: as any romantic Hollywood rag could clarify, marriage is about far more than an impersonal transaction involving gametes and a nice warm house in which to grow 2.1 little boogers until voting age. Rather, marriage is about people -- people who care about and are committed to one another.
Which begs the real question, then: does marriage actually serve a biological purpose or is it simply cultural contrivance? Although there is almost as much debate on the subject as there are traditions involving it, consensus is that marriage emerged several thousand years ago (back when the Judeo-Christian God was seeding the planet with dinosaur bones if you subscribe to that notion, or shortly after the emergence of private property and possibly agriculture, if you don't). Not miring ourselves in the myriad flavors of polygamy, polygyny, matriarchal polyandry, nor tribal endogamies, nor suffering any sweeping generalizations, we can identify the most modern Western incarnation of marriage to have evolved between 2-300 years ago. Before this (and to a large degree during), marriages served the principal purpose of consolidating wealth or sociopolitical power, secondarily to produce children, and finally, if one was lucky (or poor), for personal romantic reasons.
While this doesn't tell us much about what marriage isn't, on some ethnological level it does indicate what it is: people like sex. And people like people. It's the story of mankind's victory over its humble and nomadic beginnings that together we are capable of more than we are on our own. However we choose to engage these concurrent drives is under large influence of our cultures and places within it, but one thing can be certain: marriage is not innate, and is but one of many of the artifices we use to navigate the human currents.
While the Galloways' argument may be valid -- that only a man and a woman can produce a human baby -- it's an entirely semantic premise to which I doubt that they're prepared to demonstrate their commitment.
And they shouldn't have to; they're not hurting me.