I'm just about the least religious man I know and I don't believe in the marriage concept.
But it's an old concept and the government have it's reasons to motivate people to stick to it, they're trying to encourage the formation of the family cell.
That's why marriage shouldn't be touched, the government want to support it as it is and not as some arbitrary notion that everyone changes every week.
Personally I wouldn't mind giving other types of couples similar benefits but I see no obligation from the government to give it to them, they might as well let people marry animals and have the people claim it's their right and so on.
I really don't understand why people think that someone owes them that right, there are thousends of government definitions to people and situations that many don't fall into them and don't get their benefits, yet on this subject people think they can change something whenever they like.