The problem, it seems to me, is that the word "marriage" has been a religious concept also enshrined in our legal system for so long. So many laws and regulations use the word "marriage" ( originally a religious term) for what has become, de facto, a civil relationship defined in law, with various privileges, contractual obligations, and rights attached to it by statute. Given the number of statutes, contracts, etc. that would have to be amended to provide the same benefits and responsibilities to a civil union that was called that, as opposed to a "marriage", it would seem more practical to call any such civil relationship by the name "marriage", whether religiously sanctioned or not. In all honesty, I hardly think extending the right to such a relationship to a small minority of the population at large is going to have any serious negative impact on either the nation, or the traditional institution of marriage for those of us who take it in a religious context (as I do). If, for instance, Bod and her wife choose to enter into a marriage as currently sanctioned by the state (with or without the sanction of the religion of their choice), I fail to see where that affects me or my rights at all, so long as any religious institution which refuses to honor or perform such a "marriage" is allowed to do so (as many also do with respect to divorce, interfaith marriage, and so on). Would I rather see it called something else? Well, according to my faith, yes; but given the practical difficulties, I'm OK with the term, so long as all the responsibilities of the marriage contract (monogamy, etc.) are preserved, so that no special benefit is conferred on a particular group. In fact, since extending marriage in a civil sense would encourage more monogamy and stable relationships, I see more potential good than harm to be done here.
That said, those whose understanding of their faith requires them to take a different view, should not have the right to express that view, publicly or privately, taken away, or be in any way coerced into silence on the matter, nor should any religious institution be coerced into performing a ceremony which violates its established doctrine.