Why is this an important distinction to make? Well, as I understand it, the law doesn't merely "give" gays the "privilege" of marriage. Instead, it says that any two people of the legal age and sound mind can join, for whatever purpose. What the law really is, if I'm reading Priznivalli in the way she intends to be understood, is an acknowledgment that marriage as a legal institution is not merely about reproduction and continuing the species. Rather, it's about allowing two people to make the sort of commitment that allows them to be each other's guardians (as, for example, when one of them is lying in a hospital bed and unable to make decisions on his or her own behalf) and to pass on property in the ways each of them sees fit.
I am not, and will probably never be, a lawyer. However, I can say with confidence that, as a result of my reading and study, I know a bit more history than the average person. And I know enough history to realize that marriage, as we know it, is actually a fairly recent invention among human institutions.
Because the Church and State were inseparable in most European societies at least until the Enlightenment, the institution of marriage was codified in a way that not only specified who could be married to whom, and who could inherit what, but also ensured the propagation of the human race. At a time when people married in their early teen years, had ten kids--of whom four or five might survive into adulthood--and died not long after turning thirty, concerns about the survival of the human race, particularly in the face of such phenomena as the plague, made sense. Also, because most of Europe's population shared the same faith (as most of the world's societies were mono-religious), the Church had an interest in seeing the population increase.
Today, almost nobody thinks that the human race is in danger of dying out--unless, of course, we do something stupid, like start a real World War. If anything, most people would agree that we should slow the growth of, or cut down, the population. So there is no rationale for allowing only the sort of unions that will help to increase the number of people in the world.
Likewise, the fact that women have claimed our natural rights in much of the world invalidates at least some of the premises behind marriage, as it has been structured. As Priznivalli and others point out, so-called traditional marriages are based, to some degree or another, on misogyny. And, really, how can anyone rationalize that when conservatives--whether of the religious variety in Pakistan or the economic type in Britain and Germany--have elected women to lead their countries? I'd love to see how the same folks who support those women will tell their families, congregations, schools and other communities that women should submit to men. Will Michelle Bachmann--if, Goddess forbid, she is elected--defer to the wishes of her husband when she makes decisions about national security?
Whatever happens, I don't imagine that the nature of sexual relations within those, or any other, marriages will change. I suspect that very few couples today have sexual relations solely for the purpose of reproducing. (Maybe that has been the case for most couples throughout history.) And, as Priznivalli points out, many older couples stop having sex altogether but remain committed to each other. So, really, the rationale of enshrining a particular kind of sexual relationship in marriage never had any rational or moral basis. And that is the very reason why, contrary to the fear-mongering of so-called traditionalists, marriages based on those kinds of relationships will not be undermined by allowing people to marry whomever they want, regardless of gender.