Marriage isn't religion's to play with.
I'd agree, but all I care about is equality.
So, while I support marriage for all, I'd also support redefinition of 'marriage' in law to be 'civil partnership/union', extended to all, and the word 'marriage' to simply be a colloquialism to be used by people and churches as they see fit.
It would, at the very least, completely destroy the "marriage is a religious institution" argument, as it'd no longer be about 'marriage'.
I don't give a shit about what term is used, so long as the term is equally applicable to heterosexual and homosexual relationships. You dig?
But why? I mean, why even that? If you're getting rid of marriage - and I have no problem with that - why would you replace it with another form of legality surrounding ones relationships? It seems bizarre to me that it's even a legal thing. Why the government care if you're married or not? I think the government should see its citizens as individuals, not as a pairing or a group based on if they shag each other or not. Edit: Or, indeed, don't shag each other. Any arbitrary distinction you want. Just view people as individuals, whether they're "married" or not.
The legal ability of two people to say "we're a family" and operate in law as a singular unit is important.
To be honest, I'd want that to be extended past 2-person unions, but I don't think society is ready for 3+ person unions.