it's not restricted, it's defined the way it always has been. as you admitted yourself, changing that definition comprises a rethink in one of the more basic underpinnings of human civilization. provide us with what you feel is some sort of incentive for overhauling western society and we might give it at least a momentary thought. give us a reason why we should change something so central to our way of life.
Allowing same-sex marriage is no more of an overhaul of western society than repealing Prohibition was.
If you really wanna talk about the "underpinnings of human civilization," then I'll point out that same-sex marriages existed in Ancient Rome. It wasn't until Rome became Christian that same-sex marriage was expressly outlawed. So there's that.
As for a reason to allow same-sex marriage: All people should be treated equally under the law unless there is some good reason not to. And "because that's the way we've always done it" really isn't a good reason to marginalize anyone.
That's pretty broad. All laws are based on morality to one extent or another. I assume you were referring simply to things you consider victimless and not really every moral question or issue.
We can go with that, sure.
