Marriage has a legal significance according to the British constitution. It has a spiritual significance according to the teaching of some religions. And many ordinary people think that it has a social significance.
The legal, religious, and social meanings overlap in all kinds of ways, like chocolate eggs and the resurrection of Jesus. The Church of England believes that Vicars have the power to cast magic spells whereby perfectly ordinary bonking becomes a mystic allegory of the interrelationship of the spiritual and the physical worlds -- or, as they put it, a sacrament. The government says that if two people make a legally binding contract to stay together for life (and cede to the state the right to divvie up their possessions if they break that contract) it will give them a number of privileges with respect to taxation, access to children, pensions and so on. Ordinary People see Marriage as a great big party in which two people affirm their love in front of their friends and their family and probably gain some social status and respectability into the bargain. But that social status partly comes from being married according to the law of the land, and illegal marriages are probably not sacramental.
I guess for most people, the social aspect is the most important: when buying a cake, planning a meal and choosing a dress, they are not primarily thinking of the love that is betwixt Christ and his Church, nor of their pensions.
In a traditional Church of England wedding the Vicar reads from the book of Common Prayer, and then he and the happy couple disappear back stage to fill out the legal paperwork, while the organist plays a long, rambling voluntary and everyone shuffles awkwardly. This makes it quite clear to everyone that the Vicar is doing two things: casting an Anglican spell, but also changing the couple's status under English law. But the law has the upper hand in the arrangement. The Vicar can't confer the religious status of "marriage" on anyone who the law says can't marry. If the Leaping Order of St Beryl says that marriage between cousins is forbidden, Leaping Priests aren't obliged to marry cousins in his church; but if the Leaping Order says that the age of consent is 15, rather than 16, then he can't conduct child-marriages -- or if he does, they don't have any legal status. (I've heard of devout Dungeons and Dragons players who decide to get all their friends together, dress up as warriors and wizards, and have the 10th Level Cleric perform a ceremony according to the Melnibonean rite of Arioch. And that's very nice and very cute and very embarrassing for the in-laws, but it doesn't make them married in the eyes of the law, or in the eyes of any God apart, presumably, from Arioch.)
Since eighteen thirty something, it has been possible to have the "state" bit of the wedding without the "God" bit: to sign the legal documents in front of a civil servant, with minimal ceremony, and become married under the law. But those registry office wedding could be exceedingly clinical -- sometimes they really did take place in filing cabinet lined rooms in front of a council official and two witnesses -- so people who were not at all religious often chose to get married in churches -- or didn't bother to get married at all. (That is: they pretended to believe that their wedding had a spiritual significance, because a purely legal ceremony wouldn't perform the desired social function.) This wasn't an ideal arrangement, either from the point of view of the church or the state. So in two thousand and something, NuLab decided to let pubs, ships, hotels and parks accredit themselves as registry offices: the legal officials would come to you, carry out the legal formalities in a pretty room, along with whatever readings or songs you fancied. (At a stroke, this made non-religious weddings more attractive than religious ones, because you got to have the service and the party on the same premises.) There's currently a scheme to let people get hitched on the beach, although I suspect that wouldn't seem as romantic in Clacton as it would in Hawaii. Certainly not as warm. And in 2004, NuLab introduced civil partnerships which allowed same-sex relationships to have the same legal status as opposite sex ones, even though they were not actually called "marriages".
There are three wrinkles, however:
1: If you want a non-religious ceremony, then you have to have non-religious songs and non-religious readings. If you want God, head for the church of your choice. The state doesn't want it to be said that it's establishing a new religion in competition with the church of England.
2: There is no mechanism for a Vicar or Priest to officiate at a civil partnership even if the priest himself wishes to do so. That was implicit, I think, in the notion of "civil partnership". The state was saying "A relationship between two men and two women can have the legal status of a relationship between a man and a woman or a woman and man; and your family and friends may very well regard it as having the same social significance but its spiritual significance is none of the states business, thank you very much."
3: The Church of England is an established Church. The Queen is the Supreme Governor of the Church; the Prime Minister has final say on who's Archbishop; the Archbishop crowns the Queen and Richard Dawkins can't go on Thought for the Day, so there. Some Anglicans still take this to mean that if you are English you are automatically a member of the Church of England. It follows from this that everyone (regardless of church affiliation) has (in theory) the right to be married in their parish church; for their child to be christened there; and to be buried in the churchyard when they die.
This causes problems if, as sometimes still happens, the Vicar believes in God. As a Christian, he may not want to baptise a child whose parents are not serious about the ceremony: as a member of the Church of England he is legally obliged to do so. From time to time, someone suggests that the Prayer Book should contain a form of service in which a baby is given a name and prayers are said, but in which no-one sprinkles water on anybody. This is always interpreted as an attempted coup d'église by liberals and agnostics who want to stop the Church of England from going all religious on them. In fact, it's usually suggested by very hard line evangelicals who think that Baptism is so important that it shouldn't be treated as a mere social rite. (The next step would be to start immersing adults in paddling pools.)
Now, the so-called Liberal Democrats have recently proposed:
1: That the rule about religious readings at registry offices should be relaxed. Like all rules, it could be imposed rather officiously. A lot of people think that playing "I'm Loving Angels Instead" by Robin Williams (I looked it up) as the happy couple walked down the aisle would not automatically give rise to the creation of a theocracy.
2: That religious groups should be allowed to conduct civil partnership ceremonies if they want to.
3: Nothing else.
As a matter of fact, I do see a possible problem with this. Because of the established nature of the Church of England it is possible that if Civil Partnership service were permitted, church of England Vicars might find that they were obliged to carry them out, even if they themselves didn't agree with them.
Some people might say "So he damn well should: the law should make no concession to homophobia, or any other kind of phobia". But it seems to me that this is a different kind of question from the one about whether homophobic hoteliers ought to be allowed to insist that gay couples sleep in separate beds. It seems to me that regardless of how, or indeed if, you interpret Christian theology, the question about the spiritual significance of categories of bonking is one that religious groups have got to be able to decide for themselves. The state has no power to say that as of next Tuesday, sex between two men is an allegory of the mystic union which is betwixt Christ and his church, any more than it has power to say that as of next Tuesday, the powers of Darkness won't mind if you walk round stonehenge clockwise as well as widdershins, or that a ball hit to the boundary without bouncing will score 7 runs.
The Church of England itself will eventually have to form their own opinion of this question (the one about gay sex, I mean, not the one about cricket) and if all the bishops, appointed via the apostolic succession, learned in the Bible and under the guidance of the Holy Spirit decide that Male-Male and Female-Female relationships can be sacramental after all (or, much more likely, that regardless of what Mr Cranmer may have put in the Book of Common Prayer, they don't really believe all that sacramental gubbins and never have done) then the individual priest would have to accept that decision, regardless of what he or she happens to think. That's what you get for being the established church. You get more status, but less freedom of conscience. If you don't like it, bugger off to Rome and see how much freedom of conscience Ratzinger gives you.
If Quakers, Methodists and Unitarians want to have Civil Partnership ceremonies, or indeed cricket matches, in their churches, then none of this arises.
My own point of view, and please don't hit me, is that if two people love each other, then its a no-brainer that they should be allowed to affirm that love according to the religious traditions of their culture. On the other hand, I have some sympathy for the position which says "The state cares about sexual relationships only because they are likely to produce children. Two guys or two girls are quite welcome to live together, and the state doesn't care whether they call themselves 'Married', 'Flatmates', 'Confirmed Bachelors' or 'Special Friends'. We are not even neutral on the issue: it just doesn't come into the state's sphere of interest." If it had been down to me, we might have had gay church weddings, but no civil partnerships. But it wasn't.
However, what me and Steve H are interested in is not what has really happened in the real world, i.e. nothing whatsoever. What we are interested in is how this impacts on the Melosphere, where civilisation is always about to come to an end, and everything is either forbidden or compulsory.
In the Melosphere, "It is proposed that some churches may be permitted to marry gays" translates as "All church are now obliged to marry gays." Civilisation is under attack. "The attempt to stamp out Christianity in Britain is gathering pace". (She really said that. Really, really, really. I didn't make it up.)