I like to think that I'm reasonably consistent in my views (I'd probably stand by most things I've written on here, and that stretches back more than six years now), but on this I really have had a sea-change. Like Hugo Rifkind (paywall...) I literally cannot think of a single cogent reason why there should not be civil gay marriage. Make it so.
The only proviso that I would make here, is that I think it would be unconscionable if religious institutions were compelled to provide a gay marriage service. But there's no reason why they should be. Marriage is now primarily a secular institution. In order to 'be' married, you need to fulfil the civic obligation of signing the register. There's no need for any religious involvement at all. As far as the state is concerned, the church is an optional extra, not a requirement.
In addition, churches are already entitled to refuse to marry people. Many Catholic churches won't marry divorcees - some won't marry across ecumenical divisions. Synagogues, I believe, will only marry Jewish couples. I'm sure that similar barriers exist in other faiths. It's not much of a step to extend this right to discriminate to same sex couples. (Equally, obviously, if there are churches that want to provide gay marriage ceremonies, then they should be allowed to).
You can argue, like Dan Hannan, that this isn't a terribly important argument - the major steps in fighting for equal rights have already been taken. Sure, I can accept that. But if this isn't a major point, then where's the harm in allowing it? I agree with Graeme Archer - marriage should be for people who love each other. it's not the job of the state to determine who's allowed to benefit.