From what you wrote you're against gay marriage for no reason at all. You stated a preference for how things should be, not a reason for that preference.
And you seem strangely preoccupied with semantics. If marriage and civil partnership meant the same thing, what would be the point of having a distinction?
I think arguing that it must be called 'marriage' is petty. Redefining that word and stretching it to encompass more things is just stupid and less useful.
Sure, give them the same rights if you like, but just call it a 'civil partnership'.
If it was just about the rights, then I'd expect people to be happy with that. But it seems to be more about forcing an opinion onto others, which I disagree with.
So instead of just using the word marriage for something that IS a marriage, we have to invent another term that doesn't mean anything else than marriage, but of two people who are of the same sex. Just because we have to. And that isn't petty.
I've seen many things described as a marriage. Like special diplomatic relations between countries, business mergers, even product announcements. And I think that people can understand this. Once again, try to describe civil partnerships to somebody. What are you going to do? Say 'It's like a marriage, just don't call it so, because I don't like it'
(In the same way, I'd even describe your reason as 'religious, just there are no gods and churches and other deities involved'. We are also used to call fanboy disputes between the text editors or operating systems as religious wars. And it's a very good description.)
That's exclusive, not inclusive 'us', I believe. This thing isn't serving me well. In fact, it is serving me very badly (gay marriage isn't going to improve that very much, though any remedy is welcome).
I remember in about '06 there was a really complicated debate I mediated on the civil partnerships page. It basically boiled down to how civil partnerships were viewed; some editors were so dead set against not equating civil partnerships to marriage (in terms of them meaning essentially the same thing, just that marriage has a historical and social aspect to it).
Point is; the argument really did just boil down to your summary - there was no actual argument just lots of folded arms and grumbling.
And you seem strangely preoccupied with semantics. If marriage and civil partnership meant the same thing, what would be the point of having a distinction?