The below is a simple post by Amy Hall but chock full of helpful links. It's worth reading and following through the links.
We usually focus on secular reasons for maintaining the man/woman definition of marriage because there are plenty of publicly accessible reasons to give, and because until now, the people who have needed convincing about the definition of marriage weren’t those who would take the Bible into consideration.
But just because solid arguments can be drawn from natural revelation (i.e., by observing the world around us to discover what marriage is and the consequences of redefining it), that doesn’t mean there aren’t specifically Christian reasons for man/woman marriage that we ought to understand and appreciate as Christians. (See here and here, for example.)
And now that people like Matthew Vines are setting out to persuade Christians that God does not oppose same-sex marriage, it’s more important than ever that we think about how marriage fits into the bigger story of the Bible. It’s more than just a question of interpreting a few Greek terms and a handful of verses.
To that end, here are some thoughts from an interview with N.T. Wright:
With Christian or Jewish presuppositions, or indeed Muslim, then if you believe in what it says in Genesis 1 about God making heaven and earth—and the binaries in Genesis are so important—that heaven and earth, and sea and dry land, and so on and so on, and you end up with male and female. It’s all about God making complementary pairs which are meant to work together. The last scene in the Bible is the new heaven and the new earth, and the symbol for that is the marriage of Christ and his church. It’s not just one or two verses here and there which say this or that. It’s an entire narrative which works with this complementarity so that a male-plus-female marriage is a signpost or a signal about the goodness of the original creation and God’s intention for the eventual new heavens and new earth.
If you say that marriage now means something which would allow other such configurations, what you’re saying is actually that when we marry a man and a woman we’re not actually doing any of that stuff. This is just a convenient social arrangement and sexual arrangement and there it is . . . get on with it. It isn’t that that is the downgrading of marriage, it’s something that clearly has gone on for some time which is now poking its head above the parapet. If that’s what you thought marriage meant, then clearly we haven’t done a very good job in society as a whole and in the church in particular in teaching about just what a wonderful mystery marriage is supposed to be.
Be prepared for this to become a challenge within the church, just as it has at this church near Biola (where one of my friends used to attend) that recently changed its position on homosexuality and same-sex marriage. Don’t be caught off guard by this; start thinking about it now. Michael Brown has just written Can You Be Gay and Christian? Responding with Love and Truth to Questions about Homosexuality, and SBTS faculty produced an eBook Response to Matthew Vines. Those might be good places to start.