The definition of marriage within Christianity has also been altered dramatically over years (and I don't mean just since biblical times, but even more recent). There are denominations that are fine with gay marriage. There are also denominations (such as the one I was raised in - Southern Baptist) who more narrowly defined marriage as between one white female and one white male of similar economic background and family social status (the blacks could get married too - but only at their own church). However, I get the impression that if I acted as a county clerk and asked to look at family trees and income statements before signing, I would probably also be held in contempt.
The whole thing feels like a huge No True Scotsman - Oh, those Christians believe in gay marriage? Well, it's because they aren't real Christians. Churches can't seem to agree on anything - but each stands firm in their believe that they represent the true morality of their entire flock. I'm certainly convinced that the primary source of opposition to gay marriage is from Christian sources, but I also remain unconvinced that it's actually representative in the way that those most vociferous in crusading like to affirm it is.
I don't mean this in an obnoxious "not all Christians!" way, but in more of a general statement about why I don't take anyway who says "As a Christian, I believe..." very seriously - you believe because you believe, not because you're a Christian - it simply doesn't hold up that anything is dictated for you, since when we actually look under the microscope we find a very disparate and diverse set of social and political views with only a scant amount being absolutely and unequivocally mandated with a biblical basis (gay marriage certainly not being one).
Edit: This was kind of rambly, so as a summary I'll just say - I do not find a compelling reason why the opinion of an individual needs to be elevated above the opinion of another individual on the basis of that belief being "Christian", especially when Christians cannot seem to be able to create a compelling reason for even other Christians to believe what they do about a particular social institution.