Well, if it's changed recently, then it's not immutable is it?
Also, that ignores the fact that marriage *has* changed in meaning. It wasn't until recently, for example, that we lost the whole religious element to it (that was, anyway, bolted on in the first place).
Also I could point to the situation in the US, where marriages between people of different races were effectively illegal until only fifty years ago.
At the heart of it all throughout history has been the idea of its being between a man and a woman. Well, now that's changing as well.