(Some) people are hung up on the word "marriage". I think for the most part they're hung up on the reality of what that word represents, and if we don't call it that it's like not recognizing China. It's a way of denying reality. So basically, I think they're wrong.
However ... the fact that those people exist is the reality I have to deal with. I could pretend they don't exist and say, "Just call it marriage." But that doesn't seem to be working so well.
So what could we do that achieves equality and pisses off both sides just about equally? (Usually the hallmark of an effective solution.) The state doesn't get to use the word "marriage" for anyone.
Fundamentalists want to say, "You (the state) can't call that a marriage, because my religion says only a man and a woman can get married." Fine. You've just claimed "marriage" as a religious term, not a legal one. Then the state can't call what you have a marriage either.
So gay couples don't get to call themselves "legally married". But neither do straight couples. They all get the same civil union as everybody else. And if they can find a church willing to perform a religious ceremony for them, and they call that a marriage? Then they're married.