One of the chief objections that conservative Christians hold against gay marriage (and all things gay, including hate crime legislation) is found in Romans 1:21-30. Paul starts out talking about people who knew God, but "…neither were thankful: … and changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and four footed beasts, and creeping things. Wherefore God gave them up to uncleanness through the lusts of their own hearts, to dishonor their own bodies between themselves: … God gave them up unto vile affections: for even their women did change the natural use into that which is against nature: And likewise the men, leaving the natural use of the woman, burned in their lust one toward another: … Being filled with … fornication, wickedness, covetousness, … full of envy, murder, debate, deceit, … backbiters, … despiteful, proud, boasters, inventors of evil things, disobedient to parents … covenantbreakers, without natural affection, unmerciful: Who knowing … that they which commit such things are worthy of death…"
Let's see – who was Paul talking to? Oh, yeah, it says in v. 6-7: "Among whom are ye also the called of Jesus Christ: To all that be in Rome, beloved of God, called to be his saints."
Remember your high school history lessons? What was the culture like in Rome in A.D. 60, around the time Paul wrote this? Tacitus, who wrote a bit later and was not Christian, pretty much parroted this theme. It was a time of drunken orgies and slavery (which Paul thought was cool), steeped in the Greek culture where mentors were men who regularly had sex with the boys they were mentoring.
Meanwhile, the gay couples I know have been together for 5, 17, >4, 5, and ~28 years. The lesbian couples I know have been together 1, 12*, 13*, 28, 7, ~18, ~12*, ~22, and >13* years. (Strikethrough indicates divorce; asterix indicates children.) Doesn't sound much like "[burning] in their lust" to me. I remember the intense grief my coworker going through divorce suffered – it was a dark time, and for weeks her eyes were often red from crying.
The heterosexual couples I know who have been monogamous for that long have been so because they love each other. And it seems to me also that that kind of love wouldn't occur if one were "leaving the natural use" of their bodies.
Maybe there's more than one way to be natural?
It also seems to me that Paul is talking specifically to "the called of Jesus Christ" – and not to the general population. Is he saying that the standard to which he is calling his followers should be held to the general population?
I wonder what even Paul would advocate as a legal framework for treating LGBT people? He did, after all, appear to follow pretty closely Jesus' advice to "render unto Ceasar the things that are Ceasar's, and unto God the things that are God's."