Incest is illegal, but so too was homosexuality once. The conventional sexual ethic these days is one of total privacy between consenting adults, unless there are direct and proven harms to others as a consequence. Excepting where they can reproduce, I can’t see why an incestuous relationship wouldn’t be valid under that rule. If we’re making laws on the basis of consistent rights principles, and we’re saying that marriage is wholly about sanctifying a relationship between consenting adults, without regard to the purpose of that relationship (for example, whether it is reproductive), then I don’t see why you should prevent incestuous relationships any more than homosexual relationships.

Now, as you might guess, I’m not arguing that brothers ought to be allowed to marry (which will be reassuring to my brother), but that’s because I don’t think this is a rights issue. Marriage as a state institution serves state purposes before private ones, and opening participation should be allowed as they contribute to those purposes (one of which is promoting stable relationships, but another is reproduction). The other cases you mention that damage marriage are fair, but much more difficult to filter out (except intergenerational marriages – but I imagine that only reflects poorly on the institution now; back in the day it was probably widely accepted). Childless marriages only offend against marriage if you think reproduction is its sole and exclusive purpose – but if you do, well the whole gay marriage question becomes a bit moot!

On the wider rights question, this is where we differ, I think. For me, we should treat people equally on the basis of who they are rather than what they do – you draw the line more broadly, because you see a greater amount of what they do as integral to who they are.

So, for me, discrimination on the basis of race is manifestly wrong, and so too would be discrminating against a man just because they are attracted to men (well, unless you’re a women looking for a date, obviously). But you would see it as just as manifestly wrong to discriminate against that man for living with another man; I wouldn’t.

While I don’t have a problem with men living with other men, I also don’t discriminate against people ‘living in sin’; but I can understand that some people (not many, but some) follow moral codes that take an especially dim view of such lifestyles. You might dismiss them as religious loons, and have nothing to do with them on the basis of their actions, and openly proclaim their moral bankruptcy; but surely you should allow them to dismiss ‘practising’ homosexuals as sinners, and have nothing to do with them either, on the basis of their actions. Tolerance is demanded of us all in these matters; acceptance is not.

On your second point, of course race is different from sexuality. But, again, I don’t think that’s relevant. If one believes that gay marriage should be prohibited because it might adversely affect marriage in general by altering the perception of this social institution, surely one must also believe other forms of marriage, and individual marriages, that may similarly affect marriage in general, should also be prohibited. Interracial marriage might have had such a negative affect, and is thus a comparable form of marriage (regardless that race and sexuality are different, unless you believe that discrimination based on sexuality is valid whilst discrimination based on race is not). The same goes for intergenerational marriages, childless marriages, marriages of convenience; the list is long.

Personally, whilst I accept that marriage is not an unalienable right, I believe it is immoral and intolerable for the state to discriminate against gay people in this way. It is more than just a case of gay people losing out – by allowing this kind of discrimination, we legitimise other instances of discrimination which can or do infringe upon peoples’ human rights. That is the reason for legalising gay marriage, rather then merely offering marriage in all but name.

(Question: Only -ish?)

Re Gregg’s point, only insofar as laws are always imperfect ways of trying to ensure justice, but always have bad cases to discredit them. We have marriage licences, after all. The fact that we use general rules to limit the potential marriage combinations (which we do) is more a matter of costs and benefits. And those general rules include, for example, that a brother and sister can’t marry – even if they were both infertile, so the biological downside of incest isn’t there.

Race isn’t a simple categorical difference, certainly in biology or English law, and so is different. At any rate, race doesn’t itself alter the type of partner that you’re seeking to marry – if I change skin colour overnight, then I could still fancy the same girl; if I change sexuality, then she’s somewhat lacking in Y-chromosomes all of a sudden.

And as for banning chav marriage, I’d agree, but I think they gave up on it years ago!

"Participation is a privilege; non-participation isn’t an infringement of human rights." As Gregg points out, no-one would accept this as an defence if the law currently said that only white people could marry. Or maybe I’m just being daft again.

In more general terms it sounds like you’d accept (on human rights grounds) that gay people should be entitled to all the same material benefits (exception from inheritance tax for instance) as straight people, in which case we get "gay marriage" in all but name, a situation which perhaps we’d both be happy with.

Not sure what DKos think about the US and the ICC, and it’s far from certain that it would violate the constitution anyway. This lot, although obviously somewhat partisan, know a hell of a lot more about US law than you or I and seem to believe it would be OK…

Besides, has he questioned the validity of the US’s public debt? I thought he’d questioned the validity of using the current method long term. He’s not saying that there’s anything wrong with existing public debt; he’s saying that continuing to use the same system will cause problems with future public debt that does not currently exist. He might be wrong, but I don’t think he’s broken the Fourteenth Amendment.

If it is a matter of the value of marriage, surely individuals and individual couples must be assessed before they are allowed to join this prestigious club. If I believe Jack and Jill don’t make a good couple (because he’s forty and she’s twenty-five, because he’s black and she’s white, because they’re bound to have ugly kids, or for whatever reason), should I be able to prevent their marriage, for the good of marriage itself?

(Slogan: Ban chav marriage!)

Re arguments for gay marriage and slavery – amusing attempt to blacken an argument by association, but that’s all. Marriage IS a social institution, after all, and one that most of us want to sustain into the future (in whichever form). As a social institution, it derives its strength from what people think of it, and that will depend on who is involved and on what terms. Participation is a privilege; non-participation isn’t an infringement of human rights (otherwise, perhaps the Government guarantee to provide us all with a spouse?). So, to be denied the opportunity to marry is hardly a denial of your being, as is slavery – all the more so these days, when you can be married in everything but the law anyway. To compare the argument against gay marriage with the argument for slavery is plain daft.

Either way if the debate on gay marriage is now only down to whether allowing two men to get married will damage the institution in the minds of some marginal people I think it’s basically been won. Empirically I would offer the comment that legalising gay sex doesn’t seem to have hurt the popularity of heterosexual sex.