Humanist weddings, and why not?

Scotland leads the way again, permitting the first British "humanist wedding" to be legally recognised. The more I think about it, however, the less I understand why it's any business of the State how couples choose to commit to each other. Rather than have community conflict (as is happening in Spain at present) over "gay weddings" why doesn't the State back off altogether?

Religious people can marry in churches, temples, synagogues or mosques. For them, that will be the real marriage as for most of them it already is. Others can use whatever ceremony they like. Why should it be anyone else's business? The law really only needs to intervene when people have bought property in common (obvious enough from the title registers) and/or have children together (obvious enough from the registers of births, and capable of being proved by DNA testing when in dispute).

Let's sack the registrars, repeal the laws on licensing venues for weddings and get on with our lives. It's no-one's business but ours (and our God's if we have one) whom we choose to share our life with.