If you're after my personal opinion - I think that the whole debate about gay marriage gets muddled because there's basically this interplay between the state and religion.
So you end up in this weird place where - those who really believe in the religious aspects of marriage have to be against gay marriage because it's literally "against the rules", it doesn't compile.
But then in the real world, we have a state that confers benefits based on marriage - and even in places where civil partnerships are wholly equal in the law, you still have the weird 'status' style thing whereby they're sort of seen as being lesser.
To me it's almost completely a nomenclature thing. If we didn't have marriages or civil partnerships, and instead we asked whether two people (of arbitrary sex) should be allowed to cohabit and/or enter a contract with each other to share assets, I think it'd be way simpler.
I mean there'd still be the stalwart homophobes that just think it's capital-W Wrong or think that religiously they need to at least publically claim it as being wrong, but what I wrote up there I think is a big part of it.