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I've heard one that isn't: marriage for the purposes of procreation to ensure the continued existance of the state. (Though, there is a religious parallel to this argument, which is that God commanded us to go forth and multiply.)

However, that's a rubbish argument

Not least because it assumes that the availability of marriage is a big enough incentive to make someone switch teams. Now I guess that might have a small amount of traction in the bi community, but I'm pretty sure the asymmetrical availability of marriage hasn't been a factor in the straightness of any of the straight couples I know, nor is it a strain on the gayness of any of the gay couples I know. And heck, would anyone want to promote the sort of loveless and unhappy marriages that would be likely to arise if one or both partners are denying their sexuality to marry for society's sake? (Yes, there are people who are very happy in these sorts of marriages. But there are also lots of people who are very unhappy.)

Peter