Marriage is a political act, and not an individual choice. How you marry is a way of testifying to what city you belong to. Who defines marriage? The difficulty we are having in our generation in answering this question shows how theology shapes and drives everything.
If God created the world, and put one man and one woman in it, married them to each other, and established that as a pattern for the rest of human history, then marriage should be defined in accordance with that reality. If He did nothing of the kind, and we actually evolved out of the primordial goo, then we get to shape and define it however we would like it to go.
One other item of Christian theology has to be taken into account, and that is the reality of the fall into sin. The Christian approach to marriage in the context of mere Christendom deals with both of these realities -- the creational given of male and female, and the sinful propensity we have to hump the world. Creational sexuality and sinful sexuality are both factors.
Our laws about marriage must therefore do two things, not just one. They must honor what God has established in the first place, and they must restrain (by not honoring with the recognition of marriage) any of the other forms of sexual congress that sinful men have come up with...
... The marriage debates are a prime illustration of why governmental neutrality on basic religious issues is an impossibility. He who says A must eventually say B, and now that we are getting to the end of this seamy chain of syllogisms, we are confronted with the demand to allow homosexuals to marry. But this is not the end of it, and shows why it is so important to get down to first principles.
The secularists want to say that in addition to straights, we have a range of options with the fetching label of GLBTQ. Anybody who thinks that list of letters won't grow just isn't paying attention. Pederasty, bestiality, hetero-polygamy, hetero-polyandry, and bisexual-polyoptions are all waiting in the wings.
The reason why homosexual marriage won't end the debates (and the hate crimes of those who take up the wrong side of the debate) is that these marriage "reforms" clearly have not solved the problems of the bisexuals. With our arbitrary limitation of marital status to two and only two people, we are plainly telling the bisexual that he must choose between a heterosexual marriage or a homosexual marriage, but that he can't do both. "But I am both!" he wails . . . suppose this poor little buster wants to express all of his sexual yearnings within the holy bonds of matrimony, and the clerk down at the county courthouse, just seething with hate, won't give him a license with a place on it for three signatures. And then the Muslim guy, next in line, wants one with a place for four signatures...
... Christians can't fight this on the basis of "traditional values." The sexual traditions of humanity, considered apart from God's Word, have contained way too many child brides, harems, serial polygamists, and concubines to provide us with the appropriate guidance here.