An agnostic with whom I'm acquainted in real life asked some good questions in this post of his. I left a comment and thought it was worth sharing (of course, I could easily be mistaken about that).
It has to do with why Christians, in his perspective, so cavalierly cast aside Old Testament laws in the way they think and talk and discourse in the political arena today. It's a good question. See useful background information here.
-ought not carry much weight in the here and now-
That's not the Christian idea, really. They should carry a lot of weight, but at the same time, we are commanded as Christians to obey the law of the land - Romans 13, 1 Peter, etc. Doesn't mean we can't act to influence or change said laws, but we have to obey them in most every case. OT Israel was its own self-contained (covenant) community, but the covenant community of God now is not a nation, but a church dispersed among every nation. So it's not that they don't carry much weight, it's that another cmdmt carries more, and the OT civil laws were never meant to last forever.
That said, I can't think of an example right offhand of an OT civil law which is no longer regarded as immoral, a sin against God. But the penalties are generally different, if they exist at all. An example is adultery, a capital offense in OT Israel but one that may not cost you much more than $100 in court fees in modern America.
-those which are only relevant within an ancient cultural context (e.g. head-coverings for women, circumcision for men, kosher food for everyone). -
Well, you mixed up the head-covering thing, which is in 1 Corinthians (unless I err) with the other things. Circumcision was the sign of membership in the covenant, which in the covenant community of OT Israel meant membership in the society. But it's replaced by baptism in NT times as the sign of membership in the covenant community of God - the church.
And the food had ceremonial symbolism that is wholly fulfilled in Christ - read Hebrews and Mark 7.
-would qualify as one which Christians should support whenever given the chance-
You might be right about that. If you intend to show that Christians can be hypocritical at times, you'll have no argument from me.
I would not support this, b/c I don't think it lends itself to a culture in which the Gospel has the best environment to be spread and for-real accepted in people's hearts. Some separation of state from church is a good idea, though certainly not to the extent it is today.
I honestly confess I don't know what "buggery" is.
Cursing - how the $%+*#&%^% would you enforce that? ;-)
Divination - ditto.
Though these are hardly victimless crimes. The point is not that, but what amount of freedom is allowable, what consequences acceptable, and what cost entails what level of enforcement.
-What is the reasoning which allows you to treat these seemingly absolute moral commands once set in stone by a perfectly moral being as mere matters of personal conscience? -
Romans 13 and 1 Peter 2 are not all that specific. That is the reasoning.
As a matter of greater disclosure, this is far from a done deal among Reformed Christians. I say Reformed b/c most American evangelicals/evanjellyfish seem to fall thoughtlessly into the Pat Robertson/Dobson mold. But in the Reformed side of things, you have theonomic postmillennialists (like my friend Vox Veritatis, who leans postmil and who digs theonomy) who would indeed like to see the OT Israelite laws in place in modern society. You have libertarians like Vox Day (who's an open theist heretic, but still). You have people like me, somewhere in the middle, sorta, but who prefer more of a 2 Kingdoms model b/c they think it better fits the biblical data (though I'm not solidly convinced either way). So there's diversity of opinion here, and that is no doubt one of the reasons for your confusion. It confuses me too!
Hope that helps. As always, clarifying questions are welcome.