There are (I think) three basic hermeneutic approaches for those who want to align a pro-same sex marriage view with scripture. And, let's be fair, they may have even arrived at these views from scripture through applying these hermeneutics.
The first is the no comment hermeneutic. It says "the Bible does not address modern homosexuality, i.e. a consensual loving monogamous relationship between equal adult parties." This is true and should be recognised as a reason for caution by both sides of the debate in how scripture is applied to contemporary categories. OTOH the implication cannot be, but often is, that therefore the Bible doesn't speak to this issue through its general teaching on sexual ethics. This is because (Christian) biblical ethics, properly understood, is not a list of doos-and-don'ts and then when those doos-and-don'ts fail to cover a situation we have to make it up. Biblical ethics give us an ideal, a goal, they are teleological more than deontological, and so we do not need every situation covered in scripture as any situation that falls short of the God given goal is in effect wrong.
The second is the trajectory hermeneutic. It runs thus: "the Bible presents us with a trajectory of increasing liberation, the NT is more permissive than the OT; slavery and patriarchy are affirmed in the Bible but Christian tradition has come to realise they were wrong in light of the deeper teaching of scripture. As we have recognised the rights of slaves and women we are on a trajectory that now must embrace homosexuality." I would not accept this hermeneutic for two reasons, 1) I do not believe it recognises the consistency of scriptures teaching on the dignity/worth of all humanity and in particular of women and the oppressed; 2) While the issues of women and slavery are often paired in the NT e.g. 1 Cor 7, household codes, etc, (not least because most women of that era had little more rights or status than slaves) there is no correlation between these subjects and homosexuality. So even if we allow for a trajectory of liberation (I don't) there is no Biblical reason to make homosexuality the next step up that ladder than anything else originally disapproved of by scripture.
The third is the slippery slope hermeneutic. This is in many ways the least credible, but in fact the one most commonly heard. It goes like this: "well the Bible says no divorce, but we allow divorce, so who is to say we shouldn't allow gay marriage. and anyway we used to think beer and dancing were bad and now we all drink Bud' and go line dancing." This shows how often those who missed out on the debates the first time round interpret a biblical position on a contentious issue like divorce or alcohol as a simple and inevitable loosening of standards and don't understand the sound biblical reasoning behind it. This is worrying as it means many Bible believing Christians do think a pick and choose approach to scripture is the common practice and do not understand the complexities of biblical interpretation and application. It does not seek biblical justification for the next step but merely points to a lack of biblical justification for the last step . . . so what the hell does it matter what we do?
None of this is to say that I think this solves all arguments or deals with more philosophical and theological questions around the issue. I know that I would need to defend most of these assertions in more detail - this is just a blog post after all. What these hermeneutics illustrate to me is that we need to pull our discussion back from the argument in hand and actually have some basic discussions about what it means to faithfully interpret the scripture. It ceases to become gay marriage that divides us but a more fundamental issue of scriptural interpretation. Sloppy and slapdash approaches to biblical interpretation are not the sole preserve of the pro same sex marriage lobby but more conservative Christians are just as capable of being culpable on this. How sincere are we really about being biblical in this - or do we just want to be right?
Let me know what you think :-)