In the UK currently we're moving (slowly) toward the same sex marriage bill making it's way into law. (It's got a way to go still, but support seems to be there now at least.)
For the record, I'm glad to see the state moving toward allowing two people to be united under law, irrespective of terminology used - and with equal rights to those currently enjoyed by the uniting of a man & woman to date. But for a while now I've been thinking that the discussion is presented in an unclear fashion, which causes people to jump to polarising positions on it, without any common ground.
The way I see it is the UK has been a country with Religion and State intertwined for many, many hundreds of years - and therefore we have some constructs that exist both in law, and religion, but are conflated under one word and changing one appears to be changing them both.
The Church of England is Protestant, because Henry VIII wanted to get a divorce and the Catholics wouldn't allow him that, so we as a country "forked" our own religion that wasn't so different, but allowed divorce of marriage. Times were slightly different back then, the church had a lot more direct power in terms of laws & how people lived. We're a more diverse society today, and the church has less (direct?) involvement in our laws, although older laws are still based upon that original conflation of state and church.
I almost wonder if in this case (and others) we need to start diverging church from state and allowing the state to change things irrespective of the church, whilst still allowing the church to continue unhindered with practices it's held for many years. Seems like the best of both worlds almost - in law we are a very generic and equal society, and if you wish to belong to or align with a group that has more specific beliefs, then that is open to you as well.
Lets jump back to gay marriage here as an example then. If we define state marriage to be "the unity of two human beings, irrespective of their attributes", then everyone is equal to join with a partner in the eyes of the law, and all such partnerships are afforded the lawful benefits that come with that unity. The church (and indeed, all religions) are also then free to define religious marriage in their doctrine as they wish, so they can say that for them it's only permissible between a man and a woman. And if you wish to join with a partner following that doctrine, then you have to meet their rules.
In this circumstance, I wonder how much of it is a hangup on the word "marriage". I can see how the state wants to keep hold of it to describe partnerships between two people, and also that religions want to keep hold of it as they've been using it for years and neither wish to yield the word for the other to use. Which possibly means people think solely of this issue as just being an intertwined construct between state and religion, and not as the state giving everyone equality being a separate issue.